honor repack.usedeltabaseoffset when fetching packs

If the local receiving repository has disabled the use of delta base
offset, for example to retain compatibility with older versions of
Git that predate OFS_DELTA, we shouldn't ask for ofs-delta support
when we obtain a pack from the remote server.

[ issue noticed by Shawn Pearce ]

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-fetch-pack.c

@@ -13,6 +13,7 @@
 static int transfer_unpack_limit = -1;
 static int fetch_unpack_limit = -1;
 static int unpack_limit = 100;
+static int prefer_ofs_delta = 1;
 static struct fetch_pack_args args = {
 	/* .uploadpack = */ "git-upload-pack",
 };
@@ -200,7 +201,7 @@ static int find_common(int fd[2], unsigned char *result_sha1,
 				     (args.use_thin_pack ? " thin-pack" : ""),
 				     (args.no_progress ? " no-progress" : ""),
 				     (args.include_tag ? " include-tag" : ""),
-				     " ofs-delta");
+				     (prefer_ofs_delta ? " ofs-delta" : ""));
 		else
 			packet_write(fd[1], "want %s\n", sha1_to_hex(remote));
 		fetching++;
@@ -597,6 +598,11 @@ static struct ref *do_fetch_pack(int fd[2],
 			fprintf(stderr, "Server supports side-band\n");
 		use_sideband = 1;
 	}
+	if (server_supports("ofs-delta")) {
+		if (args.verbose)
+			fprintf(stderr, "Server supports ofs-delta\n");
+	} else
+		prefer_ofs_delta = 0;
 	if (everything_local(&ref, nr_match, match)) {
 		packet_flush(fd[1]);
 		goto all_done;
@@ -649,6 +655,11 @@ static int fetch_pack_config(const char *var, const char *value, void *cb)
 		return 0;
 	}
 
+	if (strcmp(var, "repack.usedeltabaseoffset") == 0) {
+		prefer_ofs_delta = git_config_bool(var, value);
+		return 0;
+	}
+
 	return git_default_config(var, value, cb);
 }