tests: do not use system hg if it does not have "files" command
Ancient hg does not have "hg files" so test-check-*.t will fail with
"unknown command 'files'":
$ hg files
hg: unknown command 'files'
$ hg --version
Mercurial Distributed SCM (version 2.6.2)
Test "hg files" and give up using syshg if it does not have "files" command.
Issue351: mq: qrefresh can create extra revlog entry
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ hg qinit
$ echo b > b
$ hg ci -A -m foo
adding b
$ echo cc > b
$ hg qnew -f foo.diff
$ echo b > b
$ hg qrefresh
$ hg debugindex b
rev offset length ..... linkrev nodeid p1 p2 (re)
0 0 3 ..... 0 1e88685f5dde 000000000000 000000000000 (re)