import: only adopt the importer's own commits in a git-call window

A turn's git-call window anchored to the newest default-branch commit inside it. The default branch also holds other people's pushed commits, and one can land inside a slow git call (pre-commit hooks running tests), so a teammate's commit could win and, since re-imports skip imported turns, stick. A window now only adopts a commit whose committer email is the importer's git email; with no email set, or only someone else's commit in the window, the turn falls back to the tip as before.