git_metrics: have git commands return strings

The code using this API expects strings, so always return it.

BUG=chromium:997354
TEST=CQ passes

Change-Id: Ib06b551f36d776ee1943186ce3f7b395d53551fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2096816
Reviewed-by: Allen Li <ayatane@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/sysmon/git_metrics.py b/scripts/sysmon/git_metrics.py
index f8fc290..fd55788 100644
--- a/scripts/sysmon/git_metrics.py
+++ b/scripts/sysmon/git_metrics.py
@@ -28,7 +28,7 @@
 
   def _check_output(self, args, **kwargs):
     return subprocess.check_output(
-        self._get_git_command() + list(args), **kwargs)
+        self._get_git_command() + list(args), **kwargs).decode('utf-8')
 
   def get_commit_hash(self):
     """Return commit hash string."""