Duncan Coutts authored May 29, 2018 and Alexis Williams committed Jul 14, 2018

This replaces the previous Brancher abstraction. It improves the error
messages.

There is a very slight change of behaviour: previously if a package
specified multiple source repos with different types (e.g. git/darcs)
and only one of those VCS tools was installed on the system, it could
fall back to selecting the other repo using the other tool. The new
behaviour is that it picks the repo to use deterministically, and then
simply fails if the VCS program is not available. This situation is very
rare, and arguably the new behaviour is more predictable.