Make setup_git_env() resolve .git file when $GIT_DIR is not specified

This makes reinitializing on a .git file repository work.

This is probably the only case that setup_git_env() (via
set_git_dir()) is called on a .git file. Other cases in
setup_git_dir_gently() and enter_repo() both cover .git file case
explicitly because they need to verify the target repo is valid.

Reported-by: Ximin Luo <infinity0@gmx.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

environment.c

@@ -123,14 +123,13 @@ static char *expand_namespace(const char *raw_namespace)
 
 static void setup_git_env(void)
 {
+	const char *gitfile;
+
 	git_dir = getenv(GIT_DIR_ENVIRONMENT);
-	git_dir = git_dir ? xstrdup(git_dir) : NULL;
-	if (!git_dir) {
-		git_dir = read_gitfile(DEFAULT_GIT_DIR_ENVIRONMENT);
-		git_dir = git_dir ? xstrdup(git_dir) : NULL;
-	}
 	if (!git_dir)
 		git_dir = DEFAULT_GIT_DIR_ENVIRONMENT;
+	gitfile = read_gitfile(git_dir);
+	git_dir = xstrdup(gitfile ? gitfile : git_dir);
 	git_object_dir = getenv(DB_ENVIRONMENT);
 	if (!git_object_dir) {
 		git_object_dir = xmalloc(strlen(git_dir) + 9);

t/t0001-init.sh

@@ -379,6 +379,10 @@ test_expect_success 'init with separate gitdir' '
 	test -d realgitdir/refs
 '
 
+test_expect_success 're-init on .git file' '
+	( cd newdir && git init )
+'
+
 test_expect_success 're-init to update git link' '
 	(
 	cd newdir &&