Restrict Windows Performance remedy to Windows platform.

This change restricts  https://codereview.chromium.org/348703002
improving Windows performance to Windows platform alone, allowing
users on other OSes to continue to use git hooks.

The fate of the performance issues is unclear as there is no
coresponding bug attached to the original change.

Change-Id: I3a7818dfd6469a1a666fa34debeedd1eeb005243
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3107586Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Tomasz Wiszkowski <ender@google.com>

gclient_scm.py

@@ -12,6 +12,7 @@ import errno
 import json
 import logging
 import os
+import platform
 import posixpath
 import re
 import sys
@@ -491,7 +492,7 @@ class GitWrapper(SCMWrapper):
     if revision.startswith('origin/'):
       revision = 'refs/remotes/' + revision
 
-    if managed:
+    if managed and platform.system() == 'Windows':
       self._DisableHooks()
 
     printed_path = False