Because:

* The imperative mood check is incorrectly reporting a warning when a
  commit subject is in imperative mood, but contains a word on the
  blacklist, e.g. "Remove the temporary fixes to foo"
* This is because the check is currently checking for the presence of
  the blacklist words anywhere in the string.

This change:

* Add test for the false positive case.
* Fix the issue by having the check test only the first word of the
  subject against the blacklist.

Notes:

* This may lead to some false negatives, but it's best effort, and the
  false positives are coming up relatively frequently.