skdiff: clean up isPathAbsolute check for Windows
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: e8541e15acce502a64c929015570ad1648e548cd@5004 2bbb7eff-a529-9590-31e7-b0007b416f81

metajack/skia,mrobinson/skia,mrobinson/skia,Cue/skia,Cue/skia,mrobinson/skia,metajack/skia,Cue/skia,Cue/skia,metajack/skia,metajack/skia,mrobinson/skia,mrobinson/skia