Extract "Upgrade Git for Windows" to a composite action

Although this is (currently, and in the immediately foreseeable
future) only used in one place, it's cumbersome and also something
that we've removed and reintroduced before. Making it a composite
action may make it reasonable to keep it around for next time it is
needed.

It had previously been removed based on the idea that it wasn't
clear what form it would need to be in if needed again. Now that it
is needed again, the current form as well as the form that seems
most likely for future needs is to install the latest stable
release of Git for Windows (rather than a release candidate, hard
coded version, etc.).

Note that this should still not be used habitually, as the Git for
Windows installation that ships preinstalled on a runner image
should be preferred except when insufficient.

See GitoxideLabs#1870, GitoxideLabs#1892, GitoxideLabs#1920, and the recent commit that reintroduced
this logic, for background.

Author: EliahKagan

.github/actions/upgrade-git-for-windows/action.yml

@@ -0,0 +1,49 @@
+name: upgrade-git-for-windows
+
+description: Upgrade Git for Windows to the latest stable release
+
+inputs:
+  arch:
+    description: >
+      Target architecture fragment of Git for Windows installer filename
+      (should be `32-bit`, `64-bit`, or `arm64`)
+    required: true
+
+runs:
+  using: composite
+
+  steps:
+    - name: Report version/build of Git in this runner image
+      run: git version --build-options
+      shell: pwsh
+    - name: Download and install latest stable Git for Windows release
+      env:
+        ARCH: ${{ inputs.arch }}
+        GH_TOKEN: ${{ github.token }}
+      run: |
+        $workingDir = '~/git-dl'
+        $repo = 'git-for-windows/git'
+        $pattern = "Git-*-${Env:ARCH}.exe"
+        $log = 'setup-log.txt'
+        # Inno Setup args reference: https://jrsoftware.org/ishelp/index.php?topic=setupcmdline
+        $arguments = @(
+          '/VERYSILENT',
+          '/SUPPRESSMSGBOXES',
+          '/ALLUSERS',
+          "/LOG=$log",
+          '/NORESTART',
+          '/CLOSEAPPLICATIONS',
+          '/FORCECLOSEAPPLICATIONS'
+        )
+
+        mkdir $workingDir | Out-Null
+        cd $workingDir
+        gh release download --repo $repo --pattern $pattern
+        $installer = Get-Item $pattern
+        Start-Process -FilePath $installer -ArgumentList $arguments -NoNewWindow -Wait
+
+        Get-Content -Path $log -Tail 50
+      shell: pwsh
+    - name: Report version/build of Git after upgrade
+      run: git version --build-options
+      shell: pwsh

.github/workflows/ci.yml

@@ -253,40 +253,13 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     steps:
-      - name: Report version/build of Git in this runner image
-        run: git version --build-options
-      - name: Upgrade Git for Windows to latest stable release
-        # TODO(ci): Remove this and related steps once `windows-11-arm` ships a new enough Git.
+      - name: Upgrade Git for Windows
+        # TODO(ci): Don't run this step once `windows-11-arm` ships a new enough Git.
         # The Windows 11 ARM runner still ships Git 2.48.*, so it remains affected by #1849.
         if: matrix.os == 'windows-11-arm'
-        env:
-          GH_TOKEN: ${{ github.token }}
-        run: |
-          $workingDir = '~/git-dl'
-          $repo = 'git-for-windows/git'
-          $pattern = 'Git-*-arm64.exe'
-          $log = 'setup-log.txt'
-          # Inno Setup args reference: https://jrsoftware.org/ishelp/index.php?topic=setupcmdline
-          $arguments = @(
-            '/VERYSILENT',
-            '/SUPPRESSMSGBOXES',
-            '/ALLUSERS',
-            "/LOG=$log",
-            '/NORESTART',
-            '/CLOSEAPPLICATIONS',
-            '/FORCECLOSEAPPLICATIONS'
-          )
-
-          mkdir $workingDir | Out-Null
-          cd $workingDir
-          gh release download --repo $repo --pattern $pattern
-          $installer = Get-Item $pattern
-          Start-Process -FilePath $installer -ArgumentList $arguments -NoNewWindow -Wait
-
-          Get-Content -Path $log -Tail 50
-      - name: Report version/build of Git after upgrade
-        if: matrix.os == 'windows-11-arm'
-        run: git version --build-options
+        uses: ./.github/actions/upgrade-git-for-windows
+        with:
+          arch: arm64
       - uses: actions/checkout@v4
       - name: Install rustup
         if: matrix.os == 'windows-11-arm'