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

Documentation/config.txt

@@ -664,14 +664,6 @@ color.grep::
 	`never`), never.  When set to `true` or `auto`, use color only
 	when the output is written to the terminal.  Defaults to `false`.
 
-color.grep.external::
-	The string value of this variable is passed to an external 'grep'
-	command as a command line option if match highlighting is turned
-	on.  If set to an empty string, no option is passed at all,
-	turning off coloring for external 'grep' calls; this is the default.
-	For GNU grep, set it to `--color=always` to highlight matches even
-	when a pager is used.
-
 color.grep.match::
 	Use customized color for matches.  The value of this variable
 	may be specified as in color.branch.<slot>.  It is passed using