Fix skdiff when using windows path that begins with a drive letter

Review URL: https://mail.google.com/mail/u/0/?shva=1#inbox/138d92244ca33549


git-svn-id: http://skia.googlecode.com/svn/trunk@4838 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed