Comment 0 for bug 1509250

Consider this scenario:

1. Lander has a silo with an MP in it, silo is built.

2. Lander pushes new commit to MP.

3. Train detects new commit and marks silo dirty.

4. Lander changes mind, and undoes commit with 'bzr uncommit' and 'bzr push --overwrite'

5. Commits now match again, however silo dirty state isn't cleared, and WATCH_ONLY build doesn't clear it.

Solution:

1. When marking a silo dirty, we need to record whether the silo is dirty because a new commit was found or because a conflicting silo was published.

2. WATCH_ONLY needs to grow a check to clear the dirty flag in the event that there is no longer a new commit on the MP.