Use git filters for incremental updates from a git repository.

This commit changes the git fetch algorithm to only retrieve blobs
that aren't included in the previously deployed site manifest, if
git filters are supported by the remote.

It also changes how manifest entry sizes are represented, such that
both decompressed and compressed sizes are stored. This enables
computing accurate (and repeatable) sizes even after incremental
updates.

Co-authored-by: David Leadbeater <dgl@dgl.cx>

src/pages.go

@@ -328,7 +328,7 @@ func getPage(w http.ResponseWriter, r *http.Request) error {
 		case "zstd":
 			// Set Content-Length ourselves since `http.ServeContent` only sets
 			// it if Content-Encoding is unset or if it's a range request.
-			w.Header().Set("Content-Length", strconv.FormatInt(*entry.Size, 10))
+			w.Header().Set("Content-Length", strconv.FormatInt(entry.GetCompressedSize(), 10))
 			w.Header().Set("Content-Encoding", "zstd")
 			serveEncodingCount.
 				With(prometheus.Labels{"transform": "zstd", "negotiated": "zstd"}).