Speed up presubmit cpplint checking by skipping unchanged files

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.

Committed: http://code.google.com/p/v8/source/detail?r=3368

[Mads Ager (chromium), 2009-11-26 10:05:32]

LGTM!