tests: add explicit coverage for update_hash_refs from rewriteutil
I couldn't find any evidence this is covered by tests in core (but there's a
good chance I missed it). We've seen a cute bug in that code, but first
let's just cover the cases that work correctly.
Differential Revision: https://phab.mercurial-scm.org/D11231
#require docutils gettext
$ $TESTDIR/check-gendoc de
checking for parse errors
Die Dateien werden dem Projektarchiv beim n\xc3\xa4chsten \xc3\x9cbernehmen (commit) hinzugef\xc3\xbcgt. Um dies vorher r\xc3\xbcckg\xc3\xa4ngig zu machen, siehe:hg:`forget`. (esc)
warning: please have a space before :hg: