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