object-file: move `mkdir_in_gitdir()` into "path.c"

object-file: move `mkdir_in_gitdir()` into "path.c"

The `mkdir_in_gitdir()` function is similar to `safe_create_dir()`, but
the former is hosted in "object-file.c" whereas the latter is hosted in
"path.c". The latter code unit makes way more sense though as the logic
has nothing to do with object files in particular.

Move the file into "path.c". While at it, we:

  - Rename the function to `safe_create_dir_in_gitdir()` so that the
    function names are similar to one another.

  - Remove the dependency on `the_repository` by making the callers pass
    the repository instead.

Adjust callers accordingly.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>