grep: use get_pathspec() correctly

When there is no remaining string in argv, get_pathspec(prefix, argv)
will return a two-element array that has prefix as the first element,
so there is no need to re-roll that logic in the code that uses
get_pathspec().

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/grep.c

@@ -956,13 +956,7 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 			verify_filename(prefix, argv[j]);
 	}
 
-	if (i < argc)
-		paths = get_pathspec(prefix, argv + i);
-	else if (prefix) {
-		paths = xcalloc(2, sizeof(const char *));
-		paths[0] = prefix;
-		paths[1] = NULL;
-	}
+	paths = get_pathspec(prefix, argv + i);
 	init_pathspec(&pathspec, paths);
 	pathspec.max_depth = opt.max_depth;
 	pathspec.recursive = 1;