Bug origins proposed for closed bugs #1028

Parent

#978

What to build

• Proposals: every bug issue closed in the window gets a proposed Bug origin, whoever fixed it, unless it's labelled security-fix. The first Scoring run works back through every bug issue closed so far.
• The session: a short session in Spec: Experiments, which measure a change before it reaches production #977's sandbox reads the fix and the repository's history up to it, and names the commit that introduced the bug. It names one only if the history shows it.
• The mapping: thirdshift maps that commit to its pull request, through GitHub's link from a commit to its pull requests. It then maps the pull request to its Run: the Run's Command record if this host made it, or the pull request's Built-with line if the other host did.
• A proposal counts for nothing until Confirm, correct or reject a Bug origin #1029 settles it.
• It stops at a plan-limit warning (Stopping at a plan-limit warning #1008).

Acceptance criteria

• With a fake bug fix and history, the session's named commit is mapped to its pull request and its Run.
• History that shows no introducing commit gives a proposal with no commit, never a guess.
• A security-fix bug gets no proposal.
• The first Scoring run proposes origins for every closed bug, and later ones only for their window.