Merge branch 'tb/repack-cleanup'

The recent change to "git repack" made it react less nicely when a leftover .idx file that no longer has the corresponding .pack file in the repository, which has been corrected. * tb/repack-cleanup: builtin/repack.c: avoid dir traversal in `collect_pack_filenames()` builtin/repack.c: only repack `.pack`s that exist