Instead of calling re.search() for every entry in WATCHLIST_DEFINITIONS for
every file being processed, create the regular expressions object beforehand
when parsing the watchlist file.

Processing a Chromium commit with 17k files went down from about 25 minutes
to 10 seconds with this change.

Bug: 780055
Change-Id: I6493971b67a7466ce8e1e3b28537018a724bbf47
Reviewed-on: https://chromium-review.googlesource.com/751463Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Raphael Kubo da Costa (rakuco) <raphael.kubo.da.costa@intel.com>