Add support for -F | --fixed-strings option to "git log --grep"
and friends: "git log --author", "git log --committer=<pattern>".
Code is based on implementation of this option in "git grep".
Signed-off-by: Jakub Narebski <jnareb@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>