Merge branch 'tb/grep-only-matching'

"git grep" learned the "--only-matching" option.

* tb/grep-only-matching:
  grep.c: teach 'git grep --only-matching'
  grep.c: extract show_line_header()

builtin/grep.c

@@ -844,6 +844,8 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 		OPT_BOOL_F('z', "null", &opt.null_following_name,
 			   N_("print NUL after filenames"),
 			   PARSE_OPT_NOCOMPLETE),
+		OPT_BOOL('o', "only-matching", &opt.only_matching,
+			N_("show only matching parts of a line")),
 		OPT_BOOL('c', "count", &opt.count,
 			N_("show the number of matches instead of matching lines")),
 		OPT__COLOR(&opt.color, N_("highlight matches")),
@@ -963,6 +965,10 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 	if (!opt.pattern_list)
 		die(_("no pattern given."));
 
+	/* --only-matching has no effect with --invert. */
+	if (opt.invert)
+		opt.only_matching = 0;
+
 	/*
 	 * We have to find "--" in a separate pass, because its presence
 	 * influences how we will parse arguments that come before it.