When failing to resolve tracking branches, we'd end up with a None

result, which would then get fed back into a git rev-parse, causing an

error.
This change avoids this situation, and ensures we don't try to generate

a link if we fail to locate a branch (since it won't have a stable SHA

to point to). It also and asserts that we don't pass in either a None

for the command or any falsey values in the list.

Unit tests pass.
Successfully built Review Board docs.