setup: do not forget working dir from subdir of gitdir

v1.6.1.3~4^2 (Fix gitdir detection when in subdir of gitdir,
2009-01-16) did not go far enough: when a git directory is
an ancestor of the original working directory, not only
should GIT_DIR be set to point to the .git directory, but
the original working directory should be restored before
carrying out the relevant command.

This way, the effect of running a git command from a subdir
of .git will be the same whether or not GIT_DIR is explicitly
set.

Noticed while investigating v1.6.0.3~1 (rehabilitate 'git
index-pack' inside the object store, 2008-10-20).
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

setup.c

@@ -369,6 +369,8 @@ static const char *setup_bare_git_dir(const char *work_tree_env,
 	if (!work_tree_env)
 		inside_work_tree = 0;
 	if (offset != len) {
+		if (chdir(cwd))
+			die_errno("Cannot come back to cwd");
 		root_len = offset_1st_component(cwd);
 		cwd[offset > root_len ? offset : root_len] = '\0';
 		set_git_dir(cwd);

t/t4111-apply-subdir.sh

@@ -97,7 +97,7 @@ test_expect_success 'apply from .git dir' '
 	test_cmp expected .git/file
 '
 
-test_expect_failure 'apply from subdir of .git dir' '
+test_expect_success 'apply from subdir of .git dir' '
 	cp postimage expected &&
 	cp preimage .git/file &&
 	cp preimage .git/objects/file