util: make nogc effective for CPython

279cd80059d4 made `util.nogc` a no-op. It was to optimize PyPy. But it slows
down CPython if many objects (like 300k+) are created.

For example, running `hg log -r .` without extensions in `hg-committed` with
14k+ obsmarkers have the following times:

  before        | after
  hg    | chg   | hg    | chg
  -----------------------------
  1.262 | 0.860 | 1.077 | 0.619 (seconds, best of 20 runs)

Therefore let's re-enable nogc for CPython.

Differential Revision: https://phab.mercurial-scm.org/D402

mercurial/util.py

@@ -964,10 +964,9 b' def nogc(func):'
     into. As a workaround, disable GC while building complex (huge)
     containers.
 
-    This garbage collector issue have been fixed in 2.7.
+    This garbage collector issue have been fixed in 2.7. But it still affect
+    CPython's performance.
     """
-    if sys.version_info >= (2, 7):
-        return func
     def wrapper(*args, **kwargs):
         gcenabled = gc.isenabled()
         gc.disable()
@@ -978,6 +977,10 b' def nogc(func):'
                 gc.enable()
     return wrapper
 
+if pycompat.ispypy:
+    # PyPy runs slower with gc disabled
+    nogc = lambda x: x
+
 def pathto(root, n1, n2):
     '''return the relative path from one place to another.
     root should use os.sep to separate directories