histedit: look up partial nodeid as partial nodeid
I'm about to remove support for repo[<partial hex nodeid>]. In the
verify() method, we know that self.node is always a partial or full
binary nodeid, so the most correct way to look up the revision is by
using changelog._partialmatch(), so let's do that. (It's closer to the
current code to do scmutil.revsymbol(), but that's less correct
because it will match a bookmark or tag that happens to have the same
prefix as the node.)
Differential Revision: https://phab.mercurial-scm.org/D3158
#require docutils gettext
$ $TESTDIR/check-gendoc it
checking for parse errors