fix: Github diffs URL

In https://github.com/NixOS/hydra/pull/1549 diffs were
offloaded to github for performance reasons.

While in some endpoints github accepts `.git` suffixed in the
repository name, in the comparison endpoint this does not seem
to be the case.

Specifically, on the main nixos org hydra this isn't working:

Example job: https://hydra.nixos.org/build/320178054

Generates a comparison link like so:
078d69f039...1cd347bf33

This just stips away the suffix and seems to work fine in local
testing.

src/root/common.tt

@@ -347,9 +347,10 @@ BLOCK renderDiffUri;
     url = res.0;
     branch = res.1;
     IF bi1.type == "hg" || bi1.type == "git" %]
-      [% IF url.substr(0, 19) == "https://github.com/" %]
+      [% IF url.substr(0, 19) == "https://github.com/";
+         github_url = url.replace('\.git$', '') %]
         <a target="_blank" [% HTML.attributes(href =>
-          url
+          github_url
           _ "/compare/"
           _ bi1.revision
           _ "..."