Merge branch 'pw/rebase-of-a-tag-fix'

Merge branch 'pw/rebase-of-a-tag-fix'

"git rebase <upstream> <tag>" failed when aborted in the middle, as
it mistakenly tried to write the tag object instead of peeling it
to HEAD.

* pw/rebase-of-a-tag-fix:
  rebase: dereference tags
  rebase: use lookup_commit_reference_by_name()
  rebase: use our standard error return value
  t3407: rework rebase --quit tests
  t3407: strengthen rebase --abort tests
  t3407: use test_path_is_missing
  t3407: rename a variable
  t3407: use test_cmp_rev
  t3407: use test_commit
  t3407: run tests in $TEST_DIRECTORY