Add a command to return the previous branch

[ibaguio]

There are use cases where we checkout a different branch, but need to do something with the previous branch that we were in. An example here would be, from branch A, we checkout branch B, but want to merge branch A to branch B.

Currently, we need to use the mouse to copy / paste the name of branch A.
Let's call this new command bp, which means branch previous.

We can then do something like:

g merge $(g bp)