Adding support for merge targets to be of a variable name.

This allows variables to be put into the field for what branch the working directory should be merged into (And pushed to later on).

Looks like GitSCMTest#testMergeFailed is failing - but I'm not sure why, when I tested a merge conflict manually it worked fine. Looks like that test was just added recently, so possibly it didn't get a thorough enough of a look over? Although it could very well be this commit - First time playing with a Jenkins plugin.

Cheers!