transplant: crash if repo.commit() finds nothing to commit (makes issue2135, issue2264 more obvious, but does nothing to fix either one) This seems to happen in two distinct cases: * patch.patch() claims success but changes nothing (e.g. the transplanted changeset adds an empty file that already exists) * patch.patch() makes changes, but repo.status() fails to report them Both of these seem like bugs in other parts of Mercurial, so arguably it's not transplant's job to detect the failure to commit. However: * detecting the problem as soon as possible is desirable * it prevents a more obscure crash later, in transplants.write() * there might be other lurking (or future) bugs that cause repo.commit() to do nothing Also, in the case of issue2264 (source changesets silently dropped by transplant), the only way to spot the problem currently is the crash in transplants.write(). Failure to transplant a patch should abort immediately, whether it's user error (patch does not apply) or a Mercurial bug (e.g. repo.status() failing to report changes).