GHA/windows: restore runtests perf with last known good Git for Windows

Use the last known good release of Git for Windows by installing it
manually. It restores `runtests.pl` performance to the levels before
the October 2024 and this week's fallouts which gradually deployed
MSYS2 runtimes with pipe/signal/concurrency issues.

Also:
- restore vcpkg job's test parallelism to `-j8` (from `-j4`).
- keep using the default shell for jobs not running tests.
  To avoid the unnecessary Git for Windows install overhead.

Upsides:
- good performance again.
- easy to experiment with any version.

Downsides:
- installing the Git for Windows package takes 15-30 seconds.
- we're pinned to an old package version.
- no canary to tell when the issue is fixed on the runner images.

Unknown:
- stability. (no MSYS2 runtimes were ever stable and it's difficult
  to quantify if a version improves or worsens stability/flakiness, and
  intermittent env failures.

Follow-up to 1bf774df57 #16217
Follow-up to 5f9411f953 #15380

Closes #16265

.github/workflows/windows.yml

@@ -360,7 +360,7 @@ jobs:
     timeout-minutes: 20
     defaults:
       run:
-        shell: C:\msys64\usr\bin\bash.exe {0}
+        shell: ${{ matrix.tflags != 'skipall' && matrix.tflags != 'skiprun' && 'C:\_gfw\usr\bin\bash.exe {0}' || 'C:\msys64\usr\bin\bash.exe {0}' }}
     strategy:
       matrix:
         include:
@@ -387,6 +387,21 @@ jobs:
             tflags: 'skiprun'
       fail-fast: false
     steps:
+      - name: 'install Git for Windows'
+        if: ${{ matrix.tflags != 'skipall' && matrix.tflags != 'skiprun' }}
+        shell: bash
+        timeout-minutes: 5
+        run: |
+          mkdir /c/_gfw
+          cd /c/_gfw || exit 1
+          # The last known good Perl version (as of v2.47.1.windows.2) without pipe/signal MSYS2
+          # runtime (?) regressions that cause runtests.pl to run at 2.5x reduced speed, is this:
+          #   https://github.com/git-for-windows/git/releases/tag/v2.46.2.windows.1
+          curl --disable --fail --silent --show-error --connect-timeout 15 --max-time 120 --retry 3 \
+            --location https://github.com/git-for-windows/git/releases/download/v2.46.2.windows.1/PortableGit-2.46.2-64-bit.7z.exe --output bin.7z
+          7z x -bd -y bin.7z
+          rm -f bin.7z
+
       - name: 'cache compiler (gcc ${{ matrix.env }})'
         uses: actions/cache@1bd1e32a3bdc45362d1e726936510720a7c30a57 # v4
         id: cache-compiler
@@ -417,6 +432,7 @@ jobs:
         timeout-minutes: 5
         run: |
           PATH="$(cygpath "${USERPROFILE}")/my-cache/${{ matrix.dir }}/bin:/c/msys64/usr/bin:$PATH"
+          [ -d /c/_gfw ] && PATH="/c/_gfw/usr/bin:$PATH"
           [ '${{ matrix.type }}' = 'Debug' ] && options+=' -DCMAKE_RUNTIME_OUTPUT_DIRECTORY_DEBUG='
           [ '${{ matrix.type }}' = 'Release' ] && options+=' -DCMAKE_RUNTIME_OUTPUT_DIRECTORY_RELEASE='
           cmake -B bld -G 'MSYS Makefiles' ${options} \
@@ -440,6 +456,7 @@ jobs:
         timeout-minutes: 5
         run: |
           PATH="$(cygpath "${USERPROFILE}")/my-cache/${{ matrix.dir }}/bin:/c/msys64/usr/bin:$PATH"
+          [ -d /c/_gfw ] && PATH="/c/_gfw/usr/bin:$PATH"
           cmake --build bld --config '${{ matrix.type }}' --parallel 5
 
       - name: 'curl version'
@@ -454,6 +471,7 @@ jobs:
         timeout-minutes: 10
         run: |
           PATH="$(cygpath "${USERPROFILE}")/my-cache/${{ matrix.dir }}/bin:/c/msys64/usr/bin:$PATH"
+          [ -d /c/_gfw ] && PATH="/c/_gfw/usr/bin:$PATH"
           cmake --build bld --config '${{ matrix.type }}' --parallel 5 --target testdeps
 
       - name: 'install test prereqs'
@@ -474,12 +492,14 @@ jobs:
             TFLAGS+=" -ac $(cygpath "${SYSTEMROOT}/System32/curl.exe")"
           fi
           PATH="$PWD/bld/lib:$PATH:/c/Program Files (x86)/stunnel/bin"
+          [ -d /c/_gfw ] && PATH="/c/_gfw/usr/bin:$PATH"
           cmake --build bld --config '${{ matrix.type }}' --target test-ci
 
       - name: 'build examples'
         timeout-minutes: 5
         run: |
           PATH="$(cygpath "${USERPROFILE}")/my-cache/${{ matrix.dir }}/bin:/c/msys64/usr/bin:$PATH"
+          [ -d /c/_gfw ] && PATH="/c/_gfw/usr/bin:$PATH"
           cmake --build bld --config '${{ matrix.type }}' --parallel 5 --target curl-examples
 
   linux-cross-mingw-w64:
@@ -567,7 +587,7 @@ jobs:
     timeout-minutes: 55
     defaults:
       run:
-        shell: C:\msys64\usr\bin\bash.exe {0}
+        shell: ${{ matrix.tflags != 'skipall' && matrix.tflags != 'skiprun' && 'C:\_gfw\usr\bin\bash.exe {0}' || 'C:\msys64\usr\bin\bash.exe {0}' }}
     env:
       VCPKG_BINARY_SOURCES: 'clear;x-gha,readwrite'
       VCPKG_DISABLE_METRICS: '1'
@@ -658,6 +678,21 @@ jobs:
 
       fail-fast: false
     steps:
+      - name: 'install Git for Windows'
+        if: ${{ matrix.tflags != 'skipall' && matrix.tflags != 'skiprun' }}
+        shell: bash
+        timeout-minutes: 5
+        run: |
+          mkdir /c/_gfw
+          cd /c/_gfw || exit 1
+          # The last known good Perl version (as of v2.47.1.windows.2) without pipe/signal MSYS2
+          # runtime (?) regressions that cause runtests.pl to run at 2.5x reduced speed, is this:
+          #   https://github.com/git-for-windows/git/releases/tag/v2.46.2.windows.1
+          curl --disable --fail --silent --show-error --connect-timeout 15 --max-time 120 --retry 3 \
+            --location https://github.com/git-for-windows/git/releases/download/v2.46.2.windows.1/PortableGit-2.46.2-64-bit.7z.exe --output bin.7z
+          7z x -bd -y bin.7z
+          rm -f bin.7z
+
       - name: 'vcpkg cache setup'
         uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7
         with:
@@ -682,7 +717,7 @@ jobs:
       - name: 'configure'
         timeout-minutes: 5
         run: |
-          PATH="/c/msys64/usr/bin:$PATH"
+          [ -d /c/_gfw ] && PATH="/c/_gfw/usr/bin:$PATH"
           if [ '${{ matrix.plat }}' = 'uwp' ]; then
             options+=' -DCMAKE_SYSTEM_NAME=WindowsStore -DCMAKE_SYSTEM_VERSION=10.0'
             cflags='-DWINAPI_FAMILY=WINAPI_FAMILY_PC_APP'
@@ -721,7 +756,7 @@ jobs:
       - name: 'build'
         timeout-minutes: 5
         run: |
-          PATH="/c/msys64/usr/bin:$PATH"
+          [ -d /c/_gfw ] && PATH="/c/_gfw/usr/bin:$PATH"
           cmake --build bld --config '${{ matrix.type }}' --parallel 5
 
       - name: 'curl version'
@@ -737,7 +772,7 @@ jobs:
         if: ${{ matrix.tflags != 'skipall' }}
         timeout-minutes: 10
         run: |
-          PATH="/c/msys64/usr/bin:$PATH"
+          [ -d /c/_gfw ] && PATH="/c/_gfw/usr/bin:$PATH"
           cmake --build bld --config '${{ matrix.type }}' --parallel 5 --target testdeps
 
       - name: 'install test prereqs'
@@ -756,18 +791,18 @@ jobs:
         if: ${{ matrix.tflags != 'skipall' && matrix.tflags != 'skiprun' }}
         timeout-minutes: 10
         run: |
-          export TFLAGS='-j4 ~WebSockets ~SCP ~612 ${{ matrix.tflags }}'
+          export TFLAGS='-j8 ~WebSockets ~SCP ~612 ${{ matrix.tflags }}'
           if [[ '${{ matrix.install }}' = *'libssh2[core,zlib]'* ]]; then
             TFLAGS+=' ~SFTP'
           elif [[ '${{ matrix.install }}' = *'libssh '* ]]; then
             TFLAGS+=' ~614'  # 'SFTP pre-quote chmod' SFTP, pre-quote, directory
           fi
           PATH="$PWD/bld/lib:$PATH:/c/Program Files (x86)/stunnel/bin:/c/Program Files/OpenSSH-Win64"
-          PATH="/c/msys64/usr/bin:$PATH"
+          [ -d /c/_gfw ] && PATH="/c/_gfw/usr/bin:$PATH"
           cmake --build bld --config '${{ matrix.type }}' --target test-ci
 
       - name: 'build examples'
         timeout-minutes: 5
         run: |
-          PATH="/c/msys64/usr/bin:$PATH"
+          [ -d /c/_gfw ] && PATH="/c/_gfw/usr/bin:$PATH"
           cmake --build bld --config '${{ matrix.type }}' --parallel 5 --target curl-examples