compare: add CompareSame()
compare: add CompareSame()

I have a use case where I'd like to know the files that are the same in the
tree, as well as the differences.

I could do this with a separate walk and excluding the paths that were
different, but since mtree is already doing all of this for me, it makes
sense to include it here. I've added a new function so that the behavior
stays the same for existing users of Compare(), since I assume mostly this
will be slower given that most files stay the same. I'd be happy to merge
it into one, though.

Signed-off-by: Tycho Andersen <6b523322678240882acbb4da5312a12d8cd0082d@tycho.ws>

vbatts/go-mtree,vbatts/go-mtree