Fix bug ID ordering in commit messages

Some commit messages contain multiple bug IDs, often from other systems
in addition to BMO.  Picking one of those IDs by accident confuses the
review system classifier.  Make sure we follow the Firefox convention and
pick the first ID found in commit message strings.

Author: mars-f

committelemetry/classifier.py

@@ -152,7 +152,14 @@ def determine_review_system(revision_json):
 
     # TODO handle multiple bugs?
     try:
-        bug_id = parse_bugs(summary).pop()
+        # Take the first bug # found.  For Firefox commits this is usually at
+        # the front of the string, like "Bug XXXXXX - fix the bar".  try to
+        # avoid messages where there is a second ID in the message, like
+        # 'Bug 1458766 [wpt PR 10812] - [LayoutNG] ...'.
+        # NOTE: Bugs with the BMO bug # at the end will still get the wrong ID,
+        # such as:
+        # '[wpt PR 10812] blah blah (bug 1111111) r=foo'
+        bug_id = parse_bugs(summary)[0]
     except IndexError:
         log.info(
             f'could not determine review system for changeset {changeset}: unable to find a bug id in the changeset summary'