Add a regular git commit alias.

[lyda]

Git users coming from svn, hg, cvs and many other places
use "ci" as an alias to "commit." I'd note that such users
are also used to "co" being an alias for "checkout" so the
"co" alias is kind of dangerous.

[alerque]

I thoroughly agree that co is a bad alias for commit. Not only is it contrary to what that command is in many other source control systems, but a log of people have git aliases for it to be checkout in git as well. It might be wise to drop that one before somebody gets burned.

[lyda]

I've done another version of this in PR #238 that retains the "co" alias. So pick whichever (or none) suits. I do think this one would be better though.

[alerque]

As much as I agree with my previous self that co is a bad alias, at this point removing it is probably more disruptive than leaving it. On that topic I think something like #241 proposes with user control over all aliases is probably a better future direction that pruning existing aliases.

As for this PR, it has been obsoleted by #238 (just merged).