Merge branch 'jc/maint-1.6.4-grep-lookahead' into jc/maint-grep-lookahead

* jc/maint-1.6.4-grep-lookahead:
grep: optimize built-in grep by skipping lines that do not hit

This needs to be an evil merge as fixmatch() changed signature since
5183bf6 (grep: Allow case insensitive search of fixed-strings,
2009-11-06).

Signed-off-by: Junio C Hamano <gitster@pobox.com>