skdiff: clean up isPathAbsolute check for Windows

This is a followup to https://codereview.appspot.com/6458046/ ('Fix skdiff when using windows path that begins with a drive letter')
Review URL: https://codereview.appspot.com/6450106

git-svn-id: http://skia.googlecode.com/svn/trunk@5004 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed