[Issue 30] Allow linking to a different branch.

[dmitrym0]

trafficstars

#30 is a feature request to allow branch name to be specified instead of the current branch. This code was written by @dotemacs; I'm just creating the PR.

To test:

1. C-u M-x git-link-different-branch

You should get a completing read to specify which branch you'd like to link to.