Merge branch 'rr/rebase-sha1-by-string-query'

Merge branch 'rr/rebase-sha1-by-string-query'

Allow various commit objects to be given to "git rebase" by ':/look
for this string' syntax, e.g. "git rebase --onto ':/there'".

* rr/rebase-sha1-by-string-query:
  rebase: use peel_committish() where appropriate
  sh-setup: add new peel_committish() helper
  t/rebase: add failing tests for a peculiar revision