clone: create intermediate directories of destination repo

The shell version used to use "mkdir -p" to create the repo
path, but the C version just calls "mkdir". Let's replicate
the old behavior. We have to create the git and worktree
leading dirs separately; while most of the time, the
worktree dir contains the git dir (as .git), the user can
override this using GIT_WORK_TREE.

We can reuse safe_create_leading_directories, but we need to
make a copy of our const buffer to do so. Since
merge-recursive uses the same pattern, we can factor this
out into a global function. This has two other cleanup
advantages for merge-recursive:

  1. mkdir_p wasn't a very good name. "mkdir -p foo/bar" actually
     creates bar, but this function just creates the leading
     directories.

  2. mkdir_p took a mode argument, but it was completely
     ignored.

Acked-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_file.c

@@ -116,6 +116,15 @@ int safe_create_leading_directories(char *path)
 	return 0;
 }
 
+int safe_create_leading_directories_const(const char *path)
+{
+	/* path points to cache entries, so xstrdup before messing with it */
+	char *buf = xstrdup(path);
+	int result = safe_create_leading_directories(buf);
+	free(buf);
+	return result;
+}
+
 char *sha1_to_hex(const unsigned char *sha1)
 {
 	static int bufno;