index-pack: setup git repository

"git index-pack" is an independent command and does not setup git
repository while still need pack.indexversion. It may miss the
info if it is in a subdirectory of the repository.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

index-pack.c

@@ -876,7 +876,9 @@ int main(int argc, char **argv)
 	char *index_name_buf = NULL, *keep_name_buf = NULL;
 	struct pack_idx_entry **idx_objects;
 	unsigned char sha1[20];
+	int nongit = 0;
 
+	setup_git_directory_gently(&nongit);
 	git_config(git_index_pack_config, NULL);
 
 	for (i = 1; i < argc; i++) {