scripts: check_compliance: check for commit message errors

Change CheckPatch() compliance test to call checkpatch.pl directly
rather than through git diff. This has the advantage of including the
commit message in what is being checked, which is useful to catch stuff
like spurious Gerrit commit ids.

The --mailback does not seem to work correctly in this mode, but since
the output is filtered by checking the command exit code anyway, that
option is redundant, so drop that as well.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>