While dep doesn't want to encourage the use of abbreviated git commit
identifiers, they come up frequently when we parse vendoring specifiers (like
glide.yaml) of existing projects. We should give a shot at parsing these and
then expanding them to their unabbreviated form.

commitInfo returned a struct from an external package. This coupling isn't
ideal, and we wouldn't use the extra info for anything. It's better to just have
a method for exactly what we want, which is disambiguation of short revision
specifiers.

Now that we are disambiguating revisions by going to the source, we
don't need the heuristic check of string length and hex parseability
anymore.

This just cleans up the SourceMgr implementation a bit.

The previous commit changed the order of prefernce when inferring
constraints. Also, moved the bazaar GUID parsing stuff to make sure
that it receives the same preference level as revisions for other
repository types.

String parsing was a little error prone, so we'd prefer to ask the
bazaar repository whether a particular string is a valid revision ID.
To do this, we need a very slightly custom version of
disambiguateRevision for *bzrSource.

This revision also refactors TestSourceManager_InferConstraint pretty
dramatically to let it work on repositories coming from different
VCSes.