Merge branch 'jk/pack-tag-of-tag' into maint

"git pack-objects --include-tag" was taught that when we know that
we are sending an object C, we want a tag B that directly points at
C but also a tag A that points at the tag B.  We used to miss the
intermediate tag B in some cases.

* jk/pack-tag-of-tag:
  pack-objects: walk tag chains for --include-tag
  t5305: simplify packname handling
  t5305: use "git -C"
  t5305: drop "dry-run" of unpack-objects
  t5305: move cleanup into test block