tests/test-gendoc-it.t

storageutil: extract most of peek_censored from revlog

This function is super hacky and isn't correct 100% of the time. I'm going
to need this functionality on a future non-revlog store.

Let's copy things to storageutil so this code only exists once.

Differential Revision: https://phab.mercurial-scm.org/D5118

#require docutils gettext

  $ $TESTDIR/check-gendoc it
  checking for parse errors