Provide a better error message when hg upstream is empty.

Review Request #6428 — Created Oct. 10, 2014 and submitted — Latest diff uploaded

Information

RBTools
master
5d0ed62...

Reviewers

Running rbt post or rbt diff will now print an error message
that explains that it expects the upstream repository not to be
empty. The old behaviour was to print out a cryptic error message
(CRITICAL: list index out of range). This new behaviour indicates
the source of the underlying problem.

Tested rbt diff with an empty Mercurial upstream repository. It
now prints the correct error message instead of the cryptic one.