fetch-pack: add refetch

Allow a "refetch" where the contents of the local object store are
ignored and a full fetch is performed, not attempting to find or
negotiate common commits with the remote.

A key use case is to apply a new partial clone blob/tree filter and
refetch all the associated matching content, which would otherwise not
be transferred when the commit objects are already present locally.

Signed-off-by: Robert Coup <robert@coup.net.nz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fetch-pack.c

@@ -312,6 +312,7 @@ static int find_common(struct fetch_negotiator *negotiator,
 		const char *remote_hex;
 		struct object *o;
 
+		if (!args->refetch) {
 			/*
 			* If that object is complete (i.e. it is an ancestor of a
 			* local ref), we tell them we have it but do not have to
@@ -326,6 +327,7 @@ static int find_common(struct fetch_negotiator *negotiator,
 					(o->flags & COMPLETE)) {
 				continue;
 			}
+		}
 
 		remote_hex = oid_to_hex(remote);
 		if (!fetching) {
@@ -692,6 +694,9 @@ static void mark_complete_and_common_ref(struct fetch_negotiator *negotiator,
 	int old_save_commit_buffer = save_commit_buffer;
 	timestamp_t cutoff = 0;
 
+	if (args->refetch)
+		return;
+
 	save_commit_buffer = 0;
 
 	trace2_region_enter("fetch-pack", "parse_remote_refs_and_find_cutoff", NULL);
@@ -1028,7 +1033,11 @@ static struct ref *do_fetch_pack(struct fetch_pack_args *args,
 	struct fetch_negotiator *negotiator;
 
 	negotiator = &negotiator_alloc;
+	if (args->refetch) {
+		fetch_negotiator_init_noop(negotiator);
+	} else {
 		fetch_negotiator_init(r, negotiator);
+	}
 
 	sort_ref_list(&ref, ref_compare_name);
 	QSORT(sought, nr_sought, cmp_ref_by_name);
@@ -1121,7 +1130,7 @@ static struct ref *do_fetch_pack(struct fetch_pack_args *args,
 
 	mark_complete_and_common_ref(negotiator, args, &ref);
 	filter_refs(args, &ref, sought, nr_sought);
-	if (everything_local(args, &ref)) {
+	if (!args->refetch && everything_local(args, &ref)) {
 		packet_flush(fd[1]);
 		goto all_done;
 	}
@@ -1587,6 +1596,9 @@ static struct ref *do_fetch_pack_v2(struct fetch_pack_args *args,
 	struct strvec index_pack_args = STRVEC_INIT;
 
 	negotiator = &negotiator_alloc;
+	if (args->refetch)
+		fetch_negotiator_init_noop(negotiator);
+	else
 		fetch_negotiator_init(r, negotiator);
 
 	packet_reader_init(&reader, fd[0], NULL, 0,
@@ -1613,7 +1625,7 @@ static struct ref *do_fetch_pack_v2(struct fetch_pack_args *args,
 			/* Filter 'ref' by 'sought' and those that aren't local */
 			mark_complete_and_common_ref(negotiator, args, &ref);
 			filter_refs(args, &ref, sought, nr_sought);
-			if (everything_local(args, &ref))
+			if (!args->refetch && everything_local(args, &ref))
 				state = FETCH_DONE;
 			else
 				state = FETCH_SEND_REQUEST;

fetch-pack.h

@@ -42,6 +42,7 @@ struct fetch_pack_args {
 	unsigned update_shallow:1;
 	unsigned reject_shallow_remote:1;
 	unsigned deepen:1;
+	unsigned refetch:1;
 
 	/*
 	 * Indicate that the remote of this request is a promisor remote. The