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
githelp: improve help for "reset"
The previous help referenced a `hg reset`, which is a Facebook-ism.
We convert that to `hg update`. We also recognize --soft.
Differential Revision: https://phab.mercurial-scm.org/D1728