Merge branch 'jc/grep-lookahead'

* jc/grep-lookahead:
grep --no-index: allow use of "git grep" outside a git repository
grep: prepare to run outside of a work tree
grep: rip out pessimization to use fixmatch()
grep: rip out support for external grep
grep: optimize built-in grep by skipping lines that do not hit

Conflicts:
builtin-grep.c
t/t7002-grep.sh

diff --cc Documentation/config.txt
Simple merge

diff --cc Makefile
Simple merge

diff --cc builtin-grep.c
Simple merge