Misc improvements to drover
Misc improvements to drover

- Improve SVN heuristic by using cutoff of 180 days instead of 
  120. I've verified this won't cause problems with the repos
  we use.  (the closest is blink ToT and chrome, blink ToT
  refers to a revision in chrome from 9 months ago)
- Alert if using local drover.properties file. Webkit devs had
  a drover.properties file that hardcodes svn.webkit.org, which
  makes drover point to the wrong revision.

BUG=227168


Review URL: https://chromiumcodereview.appspot.com/13730015

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@192641 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools