appview,knotserver: pass repo handle to knot when proxying git ops

Adds 'x-tangled-repo-owner-handle' header when proxying git requests to
the knot. This allows knotserver to attempt to suggest the correct
git+ssh URL when rejecting a HTTP git push.

appview/state/git_http.go

···
 	// Copy original headers
 	proxyReq.Header = r.Header
 
+	repoOwnerHandle := chi.URLParam(r, "user")
+	proxyReq.Header.Add("x-tangled-repo-owner-handle", repoOwnerHandle)
+
 	// Execute request
 	resp, err := client.Do(proxyReq)
 	if err != nil {

knotserver/git.go

···
 	"io"
 	"net/http"
 	"path/filepath"
+	"strings"
 
 	securejoin "github.com/cyphar/filepath-securejoin"
 	"github.com/go-chi/chi/v5"
···
 	w.WriteHeader(http.StatusForbidden)
 
 	fmt.Fprintf(w, "Welcome to Tangled.sh!\n\nPushes are currently only supported over SSH.")
+
+	// If the appview gave us the repository owner's handle we can attempt to
+	// construct the correct ssh url.
+	ownerHandle := r.Header.Get("x-tangled-repo-owner-handle")
+	if ownerHandle != "" && !strings.ContainsAny(ownerHandle, ":") {
+		hostname := d.c.Server.Hostname
+		if strings.Contains(hostname, ":") {
+			hostname = strings.Split(hostname, ":")[0]
+		}
+
+		fmt.Fprintf(w, " Try:\ngit remote set-url --push origin git@%s:%s/%s\n\n... and push again.", hostname, ownerHandle, unqualifiedRepoName)
+	}
 	fmt.Fprintf(w, "\n\n")
 }