When indexing a non-GitHub git repo, omniglot (when the plugin is enabled) explodes with Exception("I don't know what's going on"). Let's come up with a better behavior, either by wrenching a URL out of git (if such a thing exists), or providing another option by which users can specify a URL base for another popular git-viewing tool (if one exists).
And also with Github repos that don't have an |origin| remote branch (I think that is what is happening, anyway), in that case you don't get a nice exception message - you get a NoneType for self.upstream in vcs.py and then an error that you can't |+| together NoneType and str. BTW, using upstream rather than origin is probably a better default in any case.
"origin" is a name built into the git client. "upstream" is, at best, a convention--and one I've not seen much--so I lean toward the former. However, the upstream branch name should be configurable in any case so you don't have to conform your repo to DXR's expectations.