rebase: register status file generator only once when using single transaction rebase.storestatus() behaved differently depending on whether a transaction is passed to it. If a transaction is passed, it registers a "file generator" that runs when the transaction commits. If no transaction was passed, it writes the rebase state immediately. This imprecise timing of the writing makes it hard to reason about, so let's make it more explicit which behavior we're getting by checking if we have a transaction before calling it. For the single-transaction case, move the call to storestatus(tr) early and do it only once since it's only going to write the file (at most) once anyway. Differential Revision: https://phab.mercurial-scm.org/D2912