Allow using the full GitHub URL when forking a repo.

e.g. "fork https://github.com/foo/bar as baz-plugin" is now recognised.

Makes things a bit easier when copy-pasting from the mailing list.
Don't match on single-digit issue numbers.

This is a quick fix for people asking about "jenkins-1.456.deb"
which the IRC bot would always interpret as JIRA issue "JENKINS-1".