Constrain bumps to branch

This changes the way tags are read in git, constraining the tags to just the branch currently in use.

If the tag that scmversion is trying to bump to exists, the command will error out (with a friendly message), exiting 100.

:construction: For some reason the @wip feature is not properly capturing output. I've manually verified the feature, so I am confident this works, I just can't get the cuke to pass.