git-arc.sh: add commit command

[arichardson]

Summary

This is like git arc patch, but also creates a git commit for the
phabricator revision and adjusts the commit message to drop unnecessary
tags.

Depends on D28337
Depends on D29374

[arichardson]

I'm not sure this is the best name for patch+commit, but I couldn't think of a better one.

[markj]

This assumes that arcfilter.sh lives in the same directory as git-arc, but my intent was to have git-arc be installed to /usr/local/bin. Will you do the same for arcfilter?

I'd rather see a single command rather than having both "patch" and "commit". I think the functionality you're adding can supersede "patch"'s, or we can have an option which decides whether a commit is created or not. I chose "patch" since that's what the equivalent functionality in arc is called.

I don't really like "commit" as a subcommand name, it seems too hard to intuit what that subcommand actually does. I suggest adding this functionality to "patch". "pull" might be another good subcommand name.