blackbox: add a option to duplicate output to stderr too

This is useful when debugging some performance operation.

hgext/blackbox.py

@@ -95,6 +95,13 b' configitem('
     b'track',
     default=lambda: [b'*'],
 )
+# Debug config option that also display the blackbox output on stderr
+# (in addition to writing it to disk)
+configitem(
+    b'blackbox',
+    b'debug.to-stderr',
+    default=False,
+)
 configitem(
     b'blackbox',
     b'ignore',
@@ -134,6 +141,7 b' class blackboxlogger:'
     def _log(self, ui, event, msg, opts):
         default = ui.configdate(b'devel', b'default-date')
         dateformat = ui.config(b'blackbox', b'date-format')
+        debug_to_stderr = ui.configbool(b'blackbox', b'debug.to-stderr')
         if dateformat:
             date = dateutil.datestr(default, dateformat)
         else:
@@ -165,7 +173,10 b' class blackboxlogger:'
                 maxfiles=self._maxfiles,
                 maxsize=self._maxsize,
             ) as fp:
-                fp.write(fmt % args)
+                msg = fmt % args
+                fp.write(msg)
+                if debug_to_stderr:
+                    ui.write_err(msg)
         except (IOError, OSError) as err:
             # deactivate this to avoid failed logging again
             self._trackedevents.clear()