Restrict fsmonitor warning to MacOS

R=sokcevic

Change-Id: Ie1aea3764b7e90ae0337c4a223ca6cba3ff1b499
Bug: 1475405
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4833351
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Aravind Vasudevan <aravindvasudev@google.com>

git_common.py

@@ -421,7 +421,7 @@ def warn_submodule():
   """Print warnings for submodules."""
   # TODO(crbug.com/1475405): Warn users if the project uses submodules and
   # they have fsmonitor enabled.
-  if is_fsmonitor_enabled():
+  if sys.platform.startswith('darwin') and is_fsmonitor_enabled():
     print(colorama.Fore.RED)
     print('WARNING: You have fsmonitor enabled. There is a major issue '
           'resulting in git diff-index returning wrong results. Please '