rerere: let `rerere_path()` write paths into a caller-provided buffer

rerere: let `rerere_path()` write paths into a caller-provided buffer

Same as with `get_worktree_git_dir()` a couple of commits ago, the
`rerere_path()` function returns paths that need not be free'd by the
caller because `git_path()` internally uses `get_pathname()`.

Refactor the function to instead accept a caller-provided buffer that
the path will be written into, passing on ownership to the caller. This
refactoring prepares us for the removal of `git_path()`.

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