builtin/prune.c: use strbuf to avoid having to worry about PATH_MAX
builtin/prune.c: use strbuf to avoid having to worry about PATH_MAX

While at it, rename prune_tmp_object(), which used to be a helper to
remove temporary files that were created to become loose object
files, to prune_tmp_file(), as the function is also used to remove
any random cruft whose name begins with tmp_ directly in .git/object
or .git/object/pack directories these days.

Noticed-by:  Michael Haggerty <c24c152f145c57791e809b126db9e61bd1c1782d@alum.mit.edu>
Signed-off-by: Jeff King <696e3fbcf235d40b6ea1ebd61c87cbea79d444e7@peff.net>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git