Remove assumption that index file and distribution file(s) are in the same repository

[wjwwood]

Currently bloom's pull request mechanism, which is part of the bloom-release command, assumes that the location and commit hash of the index file is the same as distribution file it is modifying. It is possible, however, that the distribution file is located in another repository or that it is located on the filesystem in which case the pull request mechanism will fail. I noticed this when coding pr #330, but fixing this assumption will take quite some more work.

[mikepurvis]

I thought I filed a bug for this ages ago, but we had this with a private rosdistro stored on bitbucket. It would be ideal if the rosdistro update step was some kind of a pluggable affair, with one to create a github PR, one for a commit to generic remote git, one to just edit a local file in place, etc.

[wjwwood]

Allowing bloom to do something different for bitbucket or local files is not what this issue is about. What I'm describing here is where the index file and the distribution file it references are in different places, e.g. in two different GitHub repositories. bloom assumes that the index file and the distribution file are in the same repository.

I believe the issue you are describing is #257.