Add git-config dependency.
Don't declare variables multiple times.
Adds some tests.

NOTE: the tests temporarily create a new .gitconfig for you. While it
should, in theory, restore your old config once it's done, you may want
to back up your .gitconfig before running these tests.
Add support for basic aliases.

Compound aliases (e.g. git ac -> git add -A && git commit) still
won't be recognized, though.