rebase: fix working copy location after a --collapse (
issue4080)
Rebasing with --collapse would leave the working copy on the parent of the
collapsed commit, instead of on the collapsed commit. This fixes that. Also
fixes a few tests that already covered this area but had bad data.
This also fixes
issue3716 where bookmarks are not kept across rebases with
--collapse. I updated the test to cover that case as well.