progress: stop getting stuck in a nested topic during a long inner step

Convert, for example, has loops like this:
for revision in source_repo:
  progress(revisions)
  for file in revision:
    progresss(file)

Prior to this change, we would start showing the file-level progress
when we encountered a big revision, and then we'd get stuck in showing
file-progress instead of revision progress, often producing many many
instantly-completing progress bars rather than the actually-helpful
top-level revisions bar.

hgext/progress.py

@@ -239,6 +239,13 class progbar(object):
             # this one are also closed
             if topic in self.topics:
                 self.topics = self.topics[:self.topics.index(topic)]
+                # reset the last topic to the one we just unwound to,
+                # so that higher-level topics will be stickier than
+                # lower-level topics
+                if self.topics:
+                    self.lasttopic = self.topics[-1]
+                else:
+                    self.lasttopic = None
         else:
             if topic not in self.topics:
                 self.starttimes[topic] = now

tests/test-progress.t

@@ -1,6 +1,14
 
   $ cat > loop.py <<EOF
   > from mercurial import commands
+  > import time
+  > class incrementingtime(object):
+  >     def __init__(self):
+  >         self._time = 0.0
+  >     def __call__(self):
+  >         self._time += 0.25
+  >         return self._time
+  > time.time = incrementingtime()
   > 
   > def loop(ui, loops, **opts):
   >     loops = int(loops)
@@ -19,9 +27,14
   >         if opts.get('parallel'):
   >             ui.progress('other', i, 'other.%d' % i, 'othernum', total)
   >         if nested:
-  >             for j in range(2):
-  >                 ui.progress('nested', j, 'nested.%d' % j, 'nestnum', 2)
-  >             ui.progress('nested', None, 'nested.done', 'nestnum', 2)
+  >             nested_steps = 2
+  >             if i and i % 4 == 0:
+  >                 nested_steps = 5
+  >             for j in range(nested_steps):
+  >                 ui.progress(
+  >                   'nested', j, 'nested.%d' % j, 'nestnum', nested_steps)
+  >             ui.progress(
+  >               'nested', None, 'nested.done', 'nestnum', nested_steps)
   >     ui.progress('loop', None, 'loop.done', 'loopnum', total)
   > 
   > commands.norepo += " loop"
@@ -69,6 +82,24 test nested short-lived topics (which sh
   loop [===============================>                ] 2/3\r (no-eol) (esc)
                                                               \r (no-eol) (esc)
 
+Test nested long-lived topic which has the same name as a short-lived
+peer. We shouldn't get stuck showing the short-lived inner steps, and
+should go back to skipping the inner steps when the slow nested step
+finishes.
+
+  $ hg -y loop 7 --nested
+  \r (no-eol) (esc)
+  loop [                                                ] 0/7\r (no-eol) (esc)
+  loop [=====>                                          ] 1/7\r (no-eol) (esc)
+  loop [============>                                   ] 2/7\r (no-eol) (esc)
+  loop [===================>                            ] 3/7\r (no-eol) (esc)
+  loop [==========================>                     ] 4/7\r (no-eol) (esc)
+  nested [==========================>                   ] 3/5\r (no-eol) (esc)
+  nested [===================================>          ] 4/5\r (no-eol) (esc)
+  loop [=================================>              ] 5/7\r (no-eol) (esc)
+  loop [========================================>       ] 6/7\r (no-eol) (esc)
+                                                              \r (no-eol) (esc)
+
 
   $ hg --config progress.changedelay=0 -y loop 3 --nested
   \r (no-eol) (esc)