Check current branch

For some reason I'm not too sure about, seveal projects have their
default branch as 'el6'.  Trying to "git checkout -t -b foo
origin/foo" when you're already on foo fails.

This adds a sanity check to avoid switching branches if already on the
right one.  This makes things more robust should upstream change their
default branch; this allows us to still specify the branch to choose
in the build part