Annotations currently link to the commit but should link to the tree - is this possible

[timdiggins]

First of all - thanks for this repo. I'm trying to get this formatter to work the way I want/expect.

Currently it seems like the annotation links to the place in the spec files (for re-running). This seems useful to me. However it currently links to the commit ie.

https://github.com/ORG/REPO/commit/HASH#annotation_36276078002
but in the context of a push this is only contains the changes in the last commit and it is not always the case (e.g. for regressions) that the spec file is part of that commit.

Ideally it would be able to have a link to the spec and line number in the tree of this, ie.

https://github.com/ORG/REPO//blob/HASH/spec/whatever_spec.rb#L678

Is this possible? I'm not fully sure how to make this happen, but would you be open to a PR if so?