Add gg alias for displaying advanced git log

I very often display the Git log graph which visualizes the DAG of the
repository. I find it to be very useful in understanding what changes
came from where and where I am in the repository history. Because I use
it so often and it is essentially a "status history", I'm assigning it
to the "gg" alias ... to match with "g" equating to "git status".
Add override for git prompt information

Occasionally, I will need to check out the code at a particular tag. It
is very helpful to know whether I am on a tag as opposed to just a
completely detached head situation. So now the prompt displays the
branch name in green if we are on a branch. And it displays the tag name,
with the prefix "tag:" if we are on a tag.