fix bench rebase tool to cope with wider range of bots.

BUG=skia:2373
R=kelvinly@google.com
TBR=kelvinly@google.com
NOTRY=true

Author: bensong@google.com

Review URL: https://codereview.chromium.org/292623005

git-svn-id: http://skia.googlecode.com/svn/trunk@14869 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed