* tools/dev/trails.py
(parse_trails_log): Specifically handle invalid input. Warn when empty
trails are encountered. Empty trails usually suggest that a non-clean
build was performed with -DSVN_FS__TRAIL_DEBUG.
(output_trail): Handle empty trails.
git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@847221 13f79535-47bb-0310-9956-ffa450edef68