Merge branch 'jk/prune-mtime' into maint

In v2.2.0, we broke "git prune" that runs in a repository that
borrows from an alternate object store.

* jk/prune-mtime:
sha1_file: fix iterating loose alternate objects
for_each_loose_file_in_objdir: take an optional strbuf path

diff --cc cache.h
Simple merge

diff --cc sha1_file.c
Simple merge