git-gc --auto: run "repack -A -d -l" as necessary.
This teaches "git-gc --auto" to consolidate many packs into one
without losing unreachable objects in them by using "repack -A"
when there are too many packfiles that are not marked with *.keep
in the repository. gc.autopacklimit configuration can be used
to set the maximum number of packs a repository is allowed to
have before this mechanism kicks in.