Grep fixes

Revert 31f5ab1 "grep -I: do not bother to read known-binary files", beacuse it is no longer necessary, see
http://article.gmane.org/gmane.comp.version-control.msysgit/20225

Amend 80d7b91 to match the variant accepted upstream, as "less -I" matches the semantics of "git grep -i".
See http://article.gmane.org/gmane.comp.version-control.git/248934