Follow-up to #2629, which stopped the walkthrough from comparing against a
branch that does not exist. The same guessing, and the same near-misses in how
the answer was applied, were left elsewhere:
- The default branch travelled as `rootBranchHint`, whose documented meaning is
"the branch the project root worktree is on". It gets its own option, because
a parameter that means two things is one the next caller gets wrong.
- A candidate equal to the branch being compared is skipped. In a plain checkout
the root hint *is* the current branch, so it won every time and produced a
comparison with itself; the repository default now wins there.
- The Changes and pull-request surfaces read the default branch too. A pull
request opened against a branch that does not exist is a worse failure than a
walkthrough that will not generate.
- `hasResolvableBaseBranch` matched `origin/feature/main` for a base of `main`,
passing the check and then failing the comparison it exists to prevent.
- `getRangeDiff` promoted only `origin/<base>`. A base carried by any other
remote stayed a bare name, which git resolves against refs/heads and nowhere
else, so it failed exactly as before.
- `getBranches` dropped every branch of a remote that did not answer, turning
"we could not ask" into "these branches are gone" — offline, that silently
removed comparisons that work fine against local remote-tracking refs.
- A remote with no `remote/HEAD` is asked once with `ls-remote --symref` rather
than falling back to the guess this data exists to replace.
The `defaultBranches` contract was documented under the status response; it
belongs to the branches response, which now has a section of its own.
Bohdan Triapitsyn
ce0e1cea27