test_hook: add a --git-dir option

The `test_hook` helper installs hook scripts into a repository.
It currently only supports `-C <dir>` to locate the target, which
triggers implicit bare repository discovery and would fail under
`safe.bareRepository=explicit`.

Add a `--git-dir <dir>` option so that the helper can locate bare
repositories explicitly. When given, the internal `git rev-parse
--absolute-git-dir` call that resolves the hooks directory uses
`--git-dir=<dir>` instead of `-C <dir>`.

See 8d1a744 (setup.c: create `safe.bareRepository`, 2022-07-14)
for the background on implicit bare repository discovery and why it may
become the default to reject it.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

t/test-lib-functions.sh

@@ -634,6 +634,8 @@ write_script () {
 #
 #   -C <dir>:
 #	Run all git commands in directory <dir>
+#   --git-dir <dir>:
+#	Use <dir> as the git directory (for bare repositories)
 #   --setup
 #	Setup a hook for subsequent tests, i.e. don't remove it in a
 #	"test_when_finished"
@@ -651,13 +653,18 @@ test_hook () {
 	disable= &&
 	remove= &&
 	indir= &&
+	gitdir= &&
 	while test $# != 0
 	do
 		case "$1" in
 		-C)
 			indir="$2" &&
 			shift
 			;;
+		--git-dir)
+			gitdir="$2" &&
+			shift
+			;;
 		--setup)
 			setup=t
 			;;
@@ -680,7 +687,8 @@ test_hook () {
 		shift
 	done &&
 
-	git_dir=$(git -C "$indir" rev-parse --absolute-git-dir) &&
+	git_dir=$(git ${indir:+-C "$indir"} ${gitdir:+--git-dir="$gitdir"} \
+		rev-parse --absolute-git-dir) &&
 	hook_dir="$git_dir/hooks" &&
 	hook_file="$hook_dir/$1" &&
 	if test -n "$disable$remove"