refs.c: make remove_empty_directories always set errno to something sane

Making errno when returning from remove_empty_directories() more
obviously meaningful, which should provide some peace of mind for
people auditing lock_ref_sha1_basic.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Acked-by: Michael Haggerty <mhagger@alum.mit.edu>

refs.c

@@ -1960,14 +1960,16 @@ static int remove_empty_directories(const char *file)
 	 * only empty directories), remove them.
 	 */
 	struct strbuf path;
-	int result;
+	int result, save_errno;
 
 	strbuf_init(&path, 20);
 	strbuf_addstr(&path, file);
 
 	result = remove_dir_recursively(&path, REMOVE_DIR_EMPTY_ONLY);
+	save_errno = errno;
 
 	strbuf_release(&path);
+	errno = save_errno;
 
 	return result;
 }
@@ -2056,6 +2058,7 @@ int dwim_log(const char *str, int len, unsigned char *sha1, char **log)
 	return logs_found;
 }
 
+/* This function should make sure errno is meaningful on error */
 static struct ref_lock *lock_ref_sha1_basic(const char *refname,
 					    const unsigned char *old_sha1,
 					    int flags, int *type_p)