Limit commits used in signature updating

Prior to the addition of x509 signature support, we only processed the
last 100 commits, so this reverts to our previous behavior by limiting
the number of commits considered to those returned by `#limited_commits`

app/services/git/branch_hooks_service.rb

@@ -112,7 +112,7 @@ module Git
     end
 
     def enqueue_update_signatures
-      unsigned = unsigned_x509_shas(commits) & unsigned_gpg_shas(commits)
+      unsigned = unsigned_x509_shas(limited_commits) & unsigned_gpg_shas(limited_commits)
       return if unsigned.empty?
 
       signable = Gitlab::Git::Commit.shas_with_signatures(project.repository, unsigned)