skdiff: if outputDir is not specified, don't write out diff files at all

skdiff: if outputDir is not specified, don't write out diff files at all.
Also adds list of files with pixel mismatch to the summary.

Sometimes we don't need the diff images, just the summary.
This will be useful for buildbots... we can run "gm -w" to generate actual
images, and run comparisons in a separate skdiff step.

BUG=http://code.google.com/p/skia/issues/detail?id=584

[bsalomon, 2012-04-26 23:50:00]

On 2012/04/26 17:31:32, epoger wrote:

LGTM

[TomH, 2012-04-27 12:56:05]

Eh - this looks like the code I'd originally wrote, but Mike requested the
two-dir default because that was the common use case.
Could we instead save the typing for all us poor backwards manual users and have
the scripts pass a flag that suppresses writing the output?

[epoger, 2012-04-27 15:55:32]

Closing in favor of https://codereview.appspot.com/6135045/ ('Add new flags to
skdiff')'