merge_tree: proper check for subdirectory

[woutdenolf]

Description

Closes #4976

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[conda-bot]

We require contributors to sign our Contributor License Agreement and we don't have one on file for @woutdenolf.

In order for us to review and merge your code, please e-sign the Contributor License Agreement PDF. We then need to manually verify your signature, merge the PR (conda/infrastructure#806), and ping the bot to refresh the PR.

[dbast]

@conda-bot check-cla

[woutdenolf]

For reference, this is a fix of another fix: #1546

[varlackc]

Looks Good

[github-actions]

Hi there, thank you for your contribution!

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed automatically if no further activity occurs.

If you would like this pull request to remain open please:

1. Rebase and verify the changes still work
2. Leave a comment with the current status

NOTE: If this pull request was closed prematurely, please leave a comment.

Thanks!

[github-actions]

Hi there, thank you for your contribution!

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed automatically if no further activity occurs.

If you would like this pull request to remain open please:

1. Rebase and verify the changes still work
2. Leave a comment with the current status

NOTE: If this pull request was closed prematurely, please leave a comment.

Thanks!

[woutdenolf]

I rebased again on main.