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

hgext/rebase.py

@@ -420,6 +420,10 b' class rebaseruntime(object):'
         # Store the state before we begin so users can run 'hg rebase --abort'
         # if we fail before the transaction closes.
         self.storestatus()
+        if tr:
+            # When using single transaction, store state when transaction
+            # commits.
+            self.storestatus(tr)
 
         cands = [k for k, v in self.state.iteritems() if v == revtodo]
         total = len(cands)
@@ -480,7 +484,8 b' class rebaseruntime(object):'
             p1, p2, base = defineparents(repo, rev, self.destmap,
                                          self.state, self.skipped,
                                          self.obsoletenotrebased)
-            self.storestatus(tr=tr)
+            if not tr:
+                self.storestatus()
             if len(repo[None].parents()) == 2:
                 repo.ui.debug('resuming interrupted rebase\n')
             else: