Fix packname hash generation.

This changes the generation of hash packfiles have in their names, from
"hash of object names as fed to us" to "hash of object names in the
resulting pack, in the order they appear in the index file".  The new
"git-index-pack" command is taught to output the computed hash value
to its standard output.

With this, we can store downloaded pack in a temporary file without
knowing its final name, run git-index-pack to generate idx for it
while finding out its final name, and then rename the pack and idx to
their final names.

Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/index-pack.c b/index-pack.c
index badbeab..785fe71 100644
--- a/index-pack.c
+++ b/index-pack.c
@@ -349,7 +349,7 @@
 	return memcmp(a->sha1, b->sha1, 20);
 }
 
-static void write_index_file(const char *index_name)
+static void write_index_file(const char *index_name, unsigned char *sha1)
 {
 	struct sha1file *f;
 	struct object_entry **sorted_by_sha =
@@ -358,6 +358,7 @@
 	struct object_entry **last = sorted_by_sha + nr_objects;
 	unsigned int array[256];
 	int i;
+	SHA_CTX ctx;
 
 	for (i = 0; i < nr_objects; ++i)
 		sorted_by_sha[i] = &objects[i];
@@ -385,6 +386,11 @@
 	}
 	sha1write(f, array, 256 * sizeof(int));
 
+	/* recompute the SHA1 hash of sorted object names.
+	 * currently pack-objects does not do this, but that
+	 * can be fixed.
+	 */
+	SHA1_Init(&ctx);
 	/*
 	 * Write the actual SHA1 entries..
 	 */
@@ -394,10 +400,12 @@
 		unsigned int offset = htonl(obj->offset);
 		sha1write(f, &offset, 4);
 		sha1write(f, obj->sha1, 20);
+		SHA1_Update(&ctx, obj->sha1, 20);
 	}
 	sha1write(f, pack_base + pack_size - 20, 20);
 	sha1close(f, NULL, 1);
 	free(sorted_by_sha);
+	SHA1_Final(sha1, &ctx);
 }
 
 int main(int argc, char **argv)
@@ -405,6 +413,7 @@
 	int i;
 	char *index_name = NULL;
 	char *index_name_buf = NULL;
+	unsigned char sha1[20];
 
 	for (i = 1; i < argc; i++) {
 		const char *arg = argv[i];
@@ -443,9 +452,11 @@
 	deltas = xcalloc(nr_objects, sizeof(struct delta_entry));
 	parse_pack_objects();
 	free(deltas);
-	write_index_file(index_name);
+	write_index_file(index_name, sha1);
 	free(objects);
 	free(index_name_buf);
 
+	printf("%s\n", sha1_to_hex(sha1));
+
 	return 0;
 }