app/appengine: don't lose repo arg when switching between branches

Fixes golang/go#36117

Change-Id: I1daf7a56bb4c8395d819e644af4b7e5bf690b7a2
Reviewed-on: https://go-review.googlesource.com/c/build/+/211138
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/app/appengine/ui.html b/app/appengine/ui.html
index a7ee7f1..da1f3cf 100644
--- a/app/appengine/ui.html
+++ b/app/appengine/ui.html
@@ -25,6 +25,7 @@
     </header>
 
     <form action="." method="GET">
+    <input type="hidden" name="repo" value="{{.Package.Path}}">
     <nav class="dashboards">
       {{if not (eq .Branch "")}}
       <label>