--inbound-branch should imply --inbound

https://bugzilla.mozilla.org/show_bug.cgi?id=1106804
I'm afraid this logic still isn't correct. We want to bisect inbound if last good revision and first bad revision are set. Otherwise we bisect nightlies, then fall back to the inbound branch specified in --inbound-branch. This conditional should be eliminated at least. some of the logic below may also need to be modified.