context: move contents of committablectx.markcommitted() to workingctx
Same reasoning as previous commits: this function updates the
dirstate. By not updating the dirstate here, we also fix the
close-head test.
Differential Revision: https://phab.mercurial-scm.org/D6365
#require docutils gettext
$ $TESTDIR/check-gendoc da
checking for parse errors