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://golang.org/cl/90080044

golang/tools,syreclabs/go-tools,josharian/tools,cw2018/tools,sshetty10/tools,jdhenke/tools,sshetty10/tools,cw2018/tools,maddyonline/tools,josharian/tools,syreclabs/go-tools,matryer/tools,sshetty10/tools,syreclabs/go-tools,nicot/tools,matryer/tools,glasser/golang-tools,springning/tools,golang/tools,runcom/tools,jdhenke/tools,nicot/tools,springning/tools,runcom/tools,dmitris/tools,springning/tools,josharian/tools,runcom/tools,nicot/tools,josharian/tools,matryer/tools,argandas/tools,matryer/tools,dmitris/tools,argandas/tools,glasser/golang-tools,jdhenke/tools,4ad/tools,runcom/tools,cw2018/tools,glasser/golang-tools,golang/tools,magiccao/tools,nicot/tools,magiccao/tools,4ad/tools,magiccao/tools,cmars/tools,maddyonline/tools,maddyonline/tools,golang/tools,syreclabs/go-tools,cmars/tools,4ad/tools,magiccao/tools,springning/tools,golang/tools,dmitris/tools,cmars/tools,cw2018/tools,maddyonline/tools,argandas/tools,glasser/golang-tools,sshetty10/tools,jdhenke/tools,argandas/tools,syreclabs/go-tools,cmars/tools,syreclabs/go-tools,dmitris/tools