strip: use in-place revset formatspec instead of %-formatting ourselves
Caught by Yuya during review of D1884.
Differential Revision: https://phab.mercurial-scm.org/D1905
revsetlang: fix a doctest example on Python 3
# skip-blame because it's just some bytes prefixes
Differential Revision: https://phab.mercurial-scm.org/D1964