Use Markdown in commit template

[steveluscher]

Commit message template in editor:

Rendering in GitHub PR:

Addresses #569.

[fanzeyi]

Pass by comment:

I think this is going to be tricky since we have tons of automation running internally using Summary: and Test Plan: as a marker.

This PR is still reasonable though. I think we might want to consider shipping a different default for open source.

[quark-zju]

How do you think about just leaving emptymsg empty? The "Summary" and "Test Plan" are Phabricator-ism that might not make sense everywhere, and the "<replace ...>" text seems to require extra key presses to delete it.