repoman: tweak commit logic wrt thin-manifest

Don't populate the "myheaders" variable when we have thin-manifests
that contain only DIST entries.
diff --git a/bin/repoman b/bin/repoman
index 6b6a197..ec227ed 100755
--- a/bin/repoman
+++ b/bin/repoman
@@ -2336,7 +2336,7 @@
 		# there's no need to regenerate manifests and all files will be
 		# committed in one big commit at the end.
 		print()
-	else:
+	elif not repo_config.thin_manifest:
 		if vcs == 'cvs':
 			headerstring = "'\$(Header|Id).*\$'"
 		elif vcs == "svn":