Completely reorganised git aliases

Here is a snapshot of my current changes in the git plugin.

This reorganizes quite deeply the way each alias is defined. They now follow a common logic:
- they are organize in group with the same prefix, some may overlap (like gc for git commit and gco for git check-out) but there is no more collision (gp for git push and gpr for git pull --rebase)
- they are easier to learn since they are no more in a single list
- more documented!

Signed-off-by: Gaetan Semet <gaetan@xeberon.net>