tracing: extract tracing-active logic to separate function

I'm about to add support for counters, and want to avoid duplicating this
logic.

Differential Revision: https://phab.mercurial-scm.org/D6525

hgdemandimport/tracing.py

@@ -13,19 +13,23 b' import os'
 _pipe = None
 _checked = False
 
[e-mail]
-def log(whencefmt, *whenceargs):
+def _isactive():
     global _pipe, _session, _checked
     if _pipe is None:
         if _checked:
-            yield
-            return
+            return False
         _checked = True
         if 'HGCATAPULTSERVERPIPE' not in os.environ:
+            return False
+        _pipe = open(os.environ['HGCATAPULTSERVERPIPE'], 'w', 1)
+        _session = os.environ.get('HGCATAPULTSESSION', 'none')
+    return True
+
[e-mail]
+def log(whencefmt, *whenceargs):
+    if not _isactive():
             yield
             return
-        _pipe = open(os.environ['HGCATAPULTSERVERPIPE'], 'w', 1)
-        _session = os.environ.get('HGCATAPULTSESSION', 'none')
     whence = whencefmt % whenceargs
     try:
         # Both writes to the pipe are wrapped in try/except to ignore