Make release build more robust to commit files

[bryevdv]

cc @mattpap When you split up the BokehJS packages, the updated files also needed to be explicitly added to the task that commits changed files to the staging branch. Otherwise the repo state is dirty when the conda build happens, and then the expected package filename is not generated. (It took me almost half a day to track down why release builds were failing in CI). I made the necessary one-off change here: 0bbb9c9

However, it would be better to update the release scripts to avoid this kind of explicit coordination, or catch the error condition earlier (before long and expensive build steps), or both. Some ideas:

• Use generic git commit . to just suck up any and all changes. I'm 👎 on this
• Assert that the repo is in a clean state after explicit commits (this is probably the minimal best thing to do)
• Have steps that modify files report those paths to Config so the commit stage can use them as input