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.
$ hg init
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls
a
unknown