doc: document git aliases for users

[Alizter]

It would be good to document how git config can have aliases which might allow some of our users to come up with their own convenient shortcuts. Since we are removing commands like gps a it would be useful to show what the proper alternative is.

Here are some of mine:

st = "status -sb";
ll = "log --oneline -40 --reverse";
last = "log -1 HEAD --stat";
cm = "commit -sm";
ci = "commit -s";
co = "checkout";
br = "branch";