Show details on ARM Git for Windows before and after upgrade

The version of Git for Windows after the upgrade (or without the
upgrade, if it is not done) is already shown in `actions/checkout`
output, since that action defaults to using the installed Git.

However, it's valuable to see information about it before and after
upgrade in the same run. It's also useful to see more details than
the version, such as to confirm the assumption that it is really
an ARM64 build.

All the information of interest could be obtained by running
`git version --build-options`. But the change here continues to
avoid running the old Git for any reason. This instead downlods
and installs `sigcheck` to inspect a (non-shim) `git.exe`.

This is with the idea that running any executables provided with
Git for Windows may increase the likelihood of being unable to
replace them, if somehow they or their subprocesses do not fully
terminate.

This is probably the wrong tradeoff, and it will probably be
changed to running `git version --build-options` before and after
the upgrade. But if that forthcoming change does cause a problem,
then it can be reverted and the approach done here used again.

Author: EliahKagan

.github/workflows/ci.yml

@@ -270,11 +270,22 @@ jobs:
         env:
           GH_TOKEN: ${{ github.token }}
         run: |
-          $workingDir = '~/git-dl'
+          # Software downloaded in this step will be placed, and run from, here.
+          $workingDir = '~/dl'
+          mkdir $workingDir | Out-Null
+          cd $workingDir
+
+          # We use Sysinternals Sigcheck to show information about the installed Git for Windows
+          # before and after upgrade. (We are not actually using it to check any signatures.)
+          curl -LO 'https://live.sysinternals.com/sigcheck64.exe'
+          Write-Output 'BEFORE upgrading Git for Windows:'
+          .\sigcheck64.exe -accepteula -nobanner 'C:\Program Files\Git\clangarm64\bin\git.exe'
+
+          # Configuration for which Git for Windows installer to download and how to run it. See
+          # https://jrsoftware.org/ishelp/index.php?topic=setupcmdline on the Inno Setup arguments.
           $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',
@@ -285,13 +296,16 @@ jobs:
             '/FORCECLOSEAPPLICATIONS'
           )
 
-          mkdir $workingDir | Out-Null
-          cd $workingDir
+          # Download the Git for Windows release from GitHub and run the Inno Setup installer.
           gh release download --repo $repo --pattern $pattern
           $installer = Get-Item $pattern
           Start-Process -FilePath $installer -ArgumentList $arguments -NoNewWindow -Wait
 
+          # Report the effect of the upgrade.
+          Write-Output 'Tail of the upgrade log:'
           Get-Content -Path $log -Tail 50
+          Write-Output 'AFTER upgrading Git for Windows:'
+          .\sigcheck64.exe -nobanner 'C:\Program Files\Git\clangarm64\bin\git.exe'
           git version
       - uses: actions/checkout@v4
       - name: Install rustup