convert: demonstrate broken {files} list in merge commits with file flags
When there is a merge in which the flags for a file from p2 is
non-empty, `hg convert` will incorrectly include that in the
changeset's files list.
Differential Revision: https://phab.mercurial-scm.org/D6408
#require docutils gettext
$ $TESTDIR/check-gendoc it
checking for parse errors