grep: Add the option '--line-number'

This is a synonym for the existing '-n' option, matching GNU grep.

Signed-off-by: Joe Ratterman <jratt0@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-grep.txt

@@ -93,6 +93,7 @@ OPTIONS
 	as a regex).
 
 -n::
+--line-number::
 	Prefix the line number to matching lines.
 
 -l::

builtin/grep.c

@@ -864,7 +864,7 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 		OPT_BOOLEAN('F', "fixed-strings", &opt.fixed,
 			"interpret patterns as fixed strings"),
 		OPT_GROUP(""),
-		OPT_BOOLEAN('n', NULL, &opt.linenum, "show line numbers"),
+		OPT_BOOLEAN('n', "line-number", &opt.linenum, "show line numbers"),
 		OPT_NEGBIT('h', NULL, &opt.pathname, "don't show filenames", 1),
 		OPT_BIT('H', NULL, &opt.pathname, "show filenames", 1),
 		OPT_NEGBIT(0, "full-name", &opt.relative,