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>

cache.h

@@ -517,6 +517,7 @@ enum sharedrepo {
 int git_config_perm(const char *var, const char *value);
 int adjust_shared_perm(const char *path);
 int safe_create_leading_directories(char *path);
+int safe_create_leading_directories_const(const char *path);
 char *enter_repo(char *path, int strict);
 static inline int is_absolute_path(const char *path)
 {