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
#require docutils gettext
$ $TESTDIR/check-gendoc it
checking for parse errors