Git commit support

[jrolfs]

• Add ghcommit filetype that inherits settings from ghhub
• Ensure diffs included in commit comment instructions are still highlighted correctly
• Update Git commentChar detection to include commit message default comment (probably can just pull that from Git settings? 😬)
• A bit of (opiniated?) code format changes

[jrolfs]

Couple things worth mentioning:

• This assumes tpop/vim-git syntax for the diff (included in vim-polyglot)
• Since this plugin technically targets hub should I supply an option to enable/disable this?

[jrolfs]

This now gracefully degrades if tpop/vim-git isn't available. Are there other Git syntax packages we should support?