progress: avoid ui.configbool() lookup when progress bar is active

Profiling revealed that the ui.configbool('progress', 'debug') during
progress bar updates was consuming a significant amount of overhead.

This commit adds an attribute on progress bar instances that caches
this config option.

The impact on `hg perfprogress` with default options is significant:

before: ! wall 4.641942 comb 4.580000 user 4.210000 sys 0.370000 (best of 3)
after:  ! wall 1.948626 comb 1.950000 user 1.950000 sys 0.000000 (best of 5)

After this change, profiling reveals that progress.progbar.progress()
is now consuming ~73% of time.

This change does not improve the execution time if the progress bar
is disabled. We may want a more comprehensive solution for that case,
as the progress bar won't be enabled in a number of scenarios (e.g.
servers and processes not attached to an interactive TTY).

I also think that overhead of ~2.0s for 1M updates is a bit high.
I suspect further refactoring of the progress bar can significantly
reduce overhead. I don't have plans to do this, however.

Differential Revision: https://phab.mercurial-scm.org/D5408

mercurial/progress.py

@@ -104,6 +104,8 b' class progbar(object):'
         self.order = self.ui.configlist('progress', 'format')
         self.estimateinterval = self.ui.configwith(
             float, 'progress', 'estimateinterval')
+        # developer config: progress.debug
+        self.debug = self.ui.configbool('progress', 'debug')
 
     def show(self, now, topic, pos, item, unit, total):
         if not shouldprint(self.ui):

mercurial/ui.py

@@ -1702,6 +1702,14 b' class ui(object):'
         elif self._progbar is not None:
             self._progbar.progress(topic, pos, item=item, unit=unit,
                                    total=total)
+
+            # Looking up progress.debug in tight loops is expensive. The value
+            # is cached on the progbar object and we can avoid the lookup in
+            # the common case where a progbar is active.
+            if pos is None or not self._progbar.debug:
+                return
+
+        # Keep this logic in sync with above.
         if pos is None or not self.configbool('progress', 'debug'):
             return