First-draft SVN revprop change handling

Fix #3100: A first attempt at having the plugin handle commit message
changes gracefully. It can happily deal with commit messages assigning
themselves to issues, removing themselves from issues, and moving
between issues. Only shortcoming at this juncture: doesn't handle
re-opening a case that was resolved by a commit that no longer applies
to an issue, that's for the next "sprint".
Merge branch 'master' of https://github.com/pantsmanuk/source-integration