test_config, test_unconfig: add a --git-dir option

In the spirit of 8d1a744 (setup.c: create `safe.bareRepository`,
2022-07-14), Git's test suite should be prepared for a potential default
change of that setting from `all` to `explicit`. The `test_config` and
`test_unconfig` helpers use `git -C <dir> config` under the hood, and
that triggers implicit bare repository discovery when <dir> is a bare
repository.

Add a `--git-dir` option alongside the existing `-C` option. When
specified, the helpers pass `--git-dir=<dir>` instead of `-C <dir>` to
the underlying `git config` call, and the `test_when_finished` cleanup
registered by `test_config` uses `test_unconfig --git-dir` accordingly.
This ensures that both the set and the unset paths bypass discovery.

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

Author: dscho

t/test-lib-functions.sh

@@ -579,13 +579,20 @@ test_modebits () {
 # Unset a configuration variable, but don't fail if it doesn't exist.
 test_unconfig () {
 	config_dir=
+	config_git_dir=
 	if test "$1" = -C
 	then
 		shift
 		config_dir=$1
 		shift
+	elif test "$1" = --git-dir
+	then
+		shift
+		config_git_dir=$1
+		shift
 	fi
-	git ${config_dir:+-C "$config_dir"} config --unset-all "$@"
+	git ${config_git_dir:+--git-dir="$config_git_dir"} \
+		${config_dir:+-C "$config_dir"} config --unset-all "$@"
 	config_status=$?
 	case "$config_status" in
 	5) # ok, nothing to unset
@@ -598,11 +605,17 @@ test_unconfig () {
 # Set git config, automatically unsetting it after the test is over.
 test_config () {
 	config_dir=
+	config_git_dir=
 	if test "$1" = -C
 	then
 		shift
 		config_dir=$1
 		shift
+	elif test "$1" = --git-dir
+	then
+		shift
+		config_git_dir=$1
+		shift
 	fi
 
 	# If --worktree is provided, use it to configure/unconfigure
@@ -613,8 +626,10 @@ test_config () {
 		shift
 	fi
 
-	test_when_finished "test_unconfig ${config_dir:+-C '$config_dir'} ${is_worktree:+--worktree} '$1'" &&
-	git ${config_dir:+-C "$config_dir"} config ${is_worktree:+--worktree} "$@"
+	test_when_finished "test_unconfig ${config_git_dir:+--git-dir '$config_git_dir'} \
+		${config_dir:+-C '$config_dir'} ${is_worktree:+--worktree} '$1'" &&
+	git ${config_git_dir:+--git-dir="$config_git_dir"} \
+		${config_dir:+-C "$config_dir"} config ${is_worktree:+--worktree} "$@"
 }
 
 test_config_global () {