seems to take an inordinate amount of time.  The idea is to
lint whole files, and only try to compute deltas if the file
contains at least one error.

For my environment, the three affected presubmit rules took
2000ms each for a 4-file changelist.  With this change,
all my rules run in about a second, and none take anything
close to 500ms.

Also, since there seem to be environment-dependent factors at
work here, I'm proposing putting timer warnings that print
a message if any check takes longer than 500ms.
Review URL: http://codereview.chromium.org/6883050

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@82559 0039d316-1c4b-4281-b951-d872f2087c98