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