A cache of MD5 sums of source file contents is now maintained. Cpplint is only invoked for new, changed, and files containing errors from the past lint check run.

As a result, repetitive presubmit checks now run in a blink of an eye, so we can include it as an obligatory pre-submit check to avoid frequent CB breakages on lint errors.

Review URL: http://codereview.chromium.org/440026

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3368 ce2b1a6d-e550-0410-aec6-3dcde31c8c00