setup_bare_git_dir(): help static analysis

Coverity reported a memory leak in this function. However, it can only
be called once, as setup_git_directory() changes global state and hence
is not reentrant.

Mark the variable as static to indicate that this is a singleton.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

setup.c

@@ -748,7 +748,7 @@ static const char *setup_bare_git_dir(struct strbuf *cwd, int offset,
 
 	/* --work-tree is set without --git-dir; use discovered one */
 	if (getenv(GIT_WORK_TREE_ENVIRONMENT) || git_work_tree_cfg) {
-		const char *gitdir;
+		static const char *gitdir;
 
 		gitdir = offset == cwd->len ? "." : xmemdupz(cwd->buf, offset);
 		if (chdir(cwd->buf))