grep: color patterns in output

Coloring matches makes them easier to spot in the output.

Add two options and two parameters: color.grep (to turn coloring on
or off), color.grep.match (to set the color of matches), --color
and --no-color (to turn coloring on or off, respectively).

The output of external greps is not changed.

This patch is based on earlier ones by Nguyễn Thái Ngọc Duy and
Thiago Alves.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -548,6 +548,15 @@ color.diff.<slot>::
 	whitespace errors). The values of these variables may be specified as
 	in color.branch.<slot>.
 
+color.grep::
+	When set to `always`, always highlight matches.  When `false` (or
+	`never`), never.  When set to `true` or `auto`, use color only
+	when the output is written to the terminal.  Defaults to `false`.
+
+color.grep.match::
+	Use customized color for matches.  The value of this variable
+	may be specified as in color.branch.<slot>.
+
 color.interactive::
 	When set to `always`, always use colors for interactive prompts
 	and displays (such as those used by "git-add --interactive").

Documentation/git-grep.txt

@@ -17,6 +17,7 @@ SYNOPSIS
 	   [-l | --files-with-matches] [-L | --files-without-match]
 	   [-z | --null]
 	   [-c | --count] [--all-match]
+	   [--color | --no-color]
 	   [-A <post-context>] [-B <pre-context>] [-C <context>]
 	   [-f <file>] [-e] <pattern>
 	   [--and|--or|--not|(|)|-e <pattern>...] [<tree>...]
@@ -105,6 +106,13 @@ OPTIONS
 	Instead of showing every matched line, show the number of
 	lines that match.
 
+--color::
+	Show colored matches.
+
+--no-color::
+	Turn off match highlighting, even when the configuration file
+	gives the default to color output.
+
 -[ABC] <context>::
 	Show `context` trailing (`A` -- after), or leading (`B`
 	-- before), or both (`C` -- context) lines, and place a