Merge branch 'jc/grep-lookahead'

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