Simplify Git hook installation

Dist/git_hooks/commit-msg.ps1

@@ -1,5 +1,4 @@
 #!/usr/bin/env -S dotnet pwsh
-# placed here by BizHawk build scripts and may be updated automatically
 $msg = Get-Content $args[0] -TotalCount 1 # this commit hook is always passed the commit message scratch file's path, so read the first line of that
 if ($msg -Match "^fix(?:ed|es)? #\d+$") {
 	echo "An issue reference alone is not a suitable commit message. Vetoed."

Dist/install_git_hooks.ps1

@@ -1,12 +1,11 @@
 $targetDir = "$PSScriptRoot/../.git/hooks"
 if (Test-Path $targetDir -PathType Container) { # is Git repo
 	$PSCommandFilename = Split-Path $PSCommandPath -Leaf
-	foreach ($f in Get-ChildItem "$PSScriptRoot/git_hook_shims") {
-		$target = Join-Path $targetDir $f.Name
+	foreach ($f in Get-ChildItem "$PSScriptRoot/git_hooks") {
+		$target = Join-Path $targetDir (Split-Path $f -LeafBase)
 		if (!(Test-Path $target -PathType Leaf)) { # target file doesn't exist
 			echo "[$PSCommandFilename] creating Git hook $($f.Name)"
-			Copy-Item $f $target
-			#TODO generate shim? the only difference between different shims would be the filename in the Batch part (and if there was an equivalent to `basename $0` then that would be the same too
+			Copy-Item "$PSScriptRoot/git_hook_shim.sh" $target
 			#TODO use symlinks on Linux
 		} elseif ($(Get-FileHash $target).Hash -ne $(Get-FileHash $f.FullName).Hash) { # files differ
 			$head = Get-Content $target -TotalCount 3