filemerge: add support for the new "mergediff" marker style to premerge
This adds support for the new "mergediff" conflict marker style to
"merge-tools.<tool>.premerge" (the new value is called
"keep-mergediff").
Differential Revision: https://phab.mercurial-scm.org/D9606
#require docutils gettext
$ $TESTDIR/check-gendoc it
checking for parse errors