scm: Replace invalid UTF-8 in Git output

This may occur when git diff is ran with binary changes in the diff.
Example: https://chromium-review.googlesource.com/c/chromium/src/+/1606595

Bug: 942522
Change-Id: I44caef559de44076ef394caa5abc3967db2bb0b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1607840
Auto-Submit: Raul Tambre <raul@tambre.ee>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>

scm.py

@@ -118,7 +118,7 @@ class GIT(object):
     env = GIT.ApplyEnvVars(kwargs)
     output = subprocess2.check_output(
         ['git'] + args, cwd=cwd, stderr=subprocess2.PIPE, env=env,
-        **kwargs).decode('utf-8')
+        **kwargs).decode('utf-8', 'replace')
     return output.strip() if strip_out else output
 
   @staticmethod