autobuild: convert top_commit_msg to str for Py 3

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>

script/autobuild.py

@@ -930,6 +930,7 @@ os.environ['TMPDIR'] = test_tmpdir
 
 # get the top commit message, for emails
 top_commit_msg = run_cmd("git log -1", dir=gitroot, output=True)
+top_commit_msg = top_commit_msg.decode('utf-8', 'backslashreplace')
 
 try:
     os.makedirs(testbase)