gc: replace local buffer with git_path

gc: replace local buffer with git_path

We probe the "17/" loose object directory for auto-gc, and
use a local buffer to format the path. We can just use
git_path() for this. It handles paths of any length
(reducing our error handling). And because we feed the
result straight to a system call, we can just use the static
variant.

Note that git_path also knows the string "objects/" is
special, and will replace it with git_object_directory()
when necessary.

Another alternative would be to use sha1_file_name() for the
pretend object "170000...", but that ends up being more
hassle for no gain, as we have to truncate the final path
component.

Signed-off-by: Jeff King <peff@peff.net>