Track when fixes land on each tree

One regression can affect both the mainline and many stable trees.
And today a regression is considered FIXED by regzbot once a fix is mainlined, before the fix reaches any of the affected stable trees.

We should track when the bugs are merged into each trees, and later, when there is a fix available, how the fix-commit propagates through the trees.

Start by reading the repository structure and the reference documentation at docs/reference.md, then trace how regzbot records regressions, mainline fixes, and stable trees. Define how each affected tree's merge status and fix-commit propagation should be represented; done means the regression is not marked fixed until the relevant tree updates are recorded.

Written by the indexing model from the issue text.