Fix timeout bug introduced in commit 9c6f735d.

This bug would cause POST hooks triggered for large repositories to
silently fail.

We need the update context to have the principal (which is tied to
the HTTP request), but not the cancellation (which is also tied to
the HTTP request and is triggered once the request is done either way).

src/pages.go

···
 		result := UpdateFromRepository(ctx, webRoot, repoURL, auth.branch)
 		resultChan <- result
 		observeSiteUpdate("webhook", &result)
-	}(r.Context())
+	}(context.WithoutCancel(r.Context()))
 
 	var result UpdateResult
 	select {