go.tools/refactor/eg: avoid nonstandard 'diff -N' option.
go.tools/refactor/eg: avoid nonstandard 'diff -N' option.

(To fix broken Solaris test)

LGTM=gri
R=gri
CC=golang-codereviews
https://codereview.appspot.com/90080044

t-yuki/go.tools.old,t-yuki/go.tools.old,t-yuki/go.tools.old