Allow GIT_DIR to be an absolute path

This fixes a problem in safe_create_leading_directories() when the
argument starts with a '/' (i.e. the path is absolute).
Signed-off-by: NJohannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

sha1_file.c

@@ -51,6 +51,8 @@ int get_sha1_hex(const char *hex, unsigned char *sha1)
 int safe_create_leading_directories(char *path)
 {
 	char *pos = path;
+	if (*pos == '/')
+		pos++;
 
 	while (pos) {
 		pos = strchr(pos, '/');