Show the deployed commit as a hover tooltip on the topbar brand

ASSET_VERSION only proves *a* deploy happened recently, not *which*
commit -- not enough to answer "did my push actually deploy, or am I
looking at a stale browser cache" (today's actual answer: neither --
deploy was current, a GeoJSON cache predating a code change was just
never invalidated, since it only checks the source .gpx's mtime).

The CI deploy job now writes REVISION (short SHA) before dropping
.git; app.rb reads it at boot, falling back to "dev" locally/in CI's
own check job where the file never exists.

views/index.erb

@@ -1,6 +1,6 @@
 <div id="app">
   <header class="topbar">
-    <span class="topbar__brand">MapTool</span>
+    <span class="topbar__brand" title="rev <%= revision %>">MapTool</span>
     <div class="topbar__auth">
       <% if logged_in %>
         <span><%= user %></span>