A test pins `revertCommit`: undoing a shipped commit keeps later changes, reports a real conflict, and refuses a merge commit with two parents #6702

src/git/revert.ts builds the commit behind every Revert. A wrong side of the three-way merge would undo other people's work. No test names revertCommit or commitParents.

Done when test/git-revert.test.ts builds a repository in $TMPDIR and shows:

• [ ] Reverting commit B with C on top undoes only B's lines, and C's change stays.
• [ ] When C rewrote B's lines, the answer is a conflict naming the file, and no ref moves.
• [ ] commitParents gives one parent for a squash, two for a merge, and none for a root. Pin what revertCommit does with each.
• [ ] The new commit's parent is ontoOid, with the given message and author.

Files: test/git-revert.test.ts, src/git/revert.ts