fix(updater): fix check for latest commit in local repository

The previous check simply compared whether the last commit of the branch
was the same in the local and the remote repository.

This commit also checks whether the remote commit is an ancestor of the
local commit. This fixes the case where the local repository has new
commits after the last published commit.

tools/check_for_upgrade.sh

@@ -75,8 +75,17 @@ function is_update_available() {
     fi
   ) || return 1
 
-  # Compare local and remote HEADs
-  [[ "$local_head" != "$remote_head" ]]
+  # Compare local and remote HEADs (if they're equal there are no updates)
+  [[ "$local_head" != "$remote_head" ]] || return 1
+
+  # If local and remote HEADs don't match, check if there's a common ancestor
+  # If the merge-base call fails, $remote_head might not be downloaded so assume there are updates
+  local base
+  base=$(cd -q "$ZSH"; git merge-base $local_head $remote_head 2>/dev/null) || return 0
+
+  # If the common ancestor ($base) is not $remote_head,
+  # the local HEAD is older than the remote HEAD
+  [[ $base != $remote_head ]]
 }
 
 function update_last_updated_file() {