[BUG] git repack fails if .git/objects/pack is on a separate partition

Console log:

Counting objects: 5137063, done. Delta compression using up to 4 threads. Compressing objects: 100% (892984/892984), done. fatal: unable to rename temporary pack file: Invalid cross-device link error: failed to run repack

Reason: write_pack_file in builtin/pack-objects.c first writes tmp file in "pack/tmp_pack_XXXXXX".

Then, it tries to rename it to "%s-%s.pack":

snprintf(tmpname, sizeof(tmpname), "%s-%s.pack",
                                 base_name, sha1_to_hex(sha1));
...
if (rename(pack_tmp_name, tmpname))
                                die_errno("unable to rename temporary pack
file");

and happily fails because that crosses device boundary. Same problem with index file. Suggested fix: do those renames inside pack directory or avoid them at all.