refs: extract out refs_create_refdir_stubs()

For Git to recognize a directory as a Git directory, it requires the
directory to contain:

  1. 'HEAD' file
  2. 'objects/' directory
  3. 'refs/' directory

Here, #1 and #3 are part of the reference storage mechanism,
specifically the files backend. Since then, newer backends such as the
reftable backend have moved to using their own path ('reftable/') for
storing references. But to ensure Git still recognizes the directory as
a Git directory, we create stubs.

There are two locations where we create stubs:

- In 'refs/reftable-backend.c' when creating the reftable backend.
- In 'clone.c' before spawning transport helpers.

In a following commit, we'll add another instance. So instead of
repeating the code, let's extract out this code to
`refs_create_refdir_stubs()` and use it.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/clone.c

@@ -1225,12 +1225,7 @@ int cmd_clone(int argc,
 	initialize_repository_version(GIT_HASH_UNKNOWN,
 				      the_repository->ref_storage_format, 1);
 
-	strbuf_addf(&buf, "%s/HEAD", git_dir);
-	write_file(buf.buf, "ref: refs/heads/.invalid");
-
-	strbuf_reset(&buf);
-	strbuf_addf(&buf, "%s/refs", git_dir);
-	safe_create_dir(the_repository, buf.buf, 1);
+	refs_create_refdir_stubs(the_repository, git_dir, NULL);
 
 	/*
 	 * additional config can be injected with -c, make sure it's included