githelp: don't reference 3rd party commands for `git show`
`hg show` is a Facebook-ism. Reference functionality in core.
The logic here isn't terrific. But it is better than nothing.
Differential Revision: https://phab.mercurial-scm.org/D1729
$ 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