Add -r<revision> option to optimize-png-fiels.sh
 When this is specified, the script process only png files
changed since this revision

Minor improvement to options handling and cygpath.

BUG=150046
NOTRY=true

Review URL: https://codereview.chromium.org/23542058

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226042 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226042