midx: allow unknown chunk ids in multi-pack index files

These chunks work like extensions where it's fine not to know what one means. We
can skip over it and keep processing the file instead of erroring out.

src/libgit2/midx.c

@@ -185,7 +185,8 @@ int git_midx_parse(
 					 chunk_oid_fanout = {0},
 					 chunk_oid_lookup = {0},
 					 chunk_object_offsets = {0},
-					 chunk_object_large_offsets = {0};
+					 chunk_object_large_offsets = {0},
+					 chunk_unknown = {0};
 
 	GIT_ASSERT_ARG(idx);
 
@@ -264,7 +265,9 @@ int git_midx_parse(
 			break;
 
 		default:
-			return midx_error("unrecognized chunk ID");
+			chunk_unknown.offset = last_chunk_offset;
+			last_chunk = &chunk_unknown;
+			break;
 		}
 	}
 	last_chunk->length = (size_t)(trailer_offset - last_chunk_offset);