Make `git full-diff` a global command

In addition, there are some upgrades to the command to make sure that it
only tries displaying color when it "should" (based on best guesses from
`git config`, and also displays untracked files which have no contents
(which would previously cause errors, or just not display anything.
Modify `rcfiles gitconfig` to remove old keys

Since the local version of `alias.full-diff` is gone in favor of the
global version, we should remove the local one from rcfiles (where it's
almost definitely going to be used), as the order of config file
precedence would mean that the old alias would supersede the new one.

Now all stale keys can be auto-removed from any given update.