Use 'backslashreplace' for decode

Resolve TODO as we are now requiring Python 3.

Change-Id: I7821627bd5c606276741c98efedaf5b11aecbcc3
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/354702
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Daniel Kutik <daniel.kutik@lavawerk.com>
diff --git a/repo b/repo
index 18649b8..8030afb 100755
--- a/repo
+++ b/repo
@@ -447,8 +447,7 @@
     except UnicodeError:
       print('repo: warning: Invalid UTF-8 output:\ncmd: %r\n%r' % (cmd, output),
             file=sys.stderr)
-      # TODO(vapier): Once we require Python 3, use 'backslashreplace'.
-      return output.decode('utf-8', 'replace')
+      return output.decode('utf-8', 'backslashreplace')
 
   # Run & package the results.
   proc = subprocess.Popen(cmd, **kwargs)