Mercurial checkouts origins contain '.hg' directory

Mercurial checkouts origins are ingested with the .hg folder. It's excluded from the nar hash computation but it remains ingested with its tree and it should not.

Original issue:

Hello!

The tarball at https://archive.softwareheritage.org/api/1/vault/flat/swh:1:dir:218d95849f10fc0691d7dfa80999ce5061e654ef/raw/ contains a `.hg` metadata directory, which I think is unintended, right?

Plan:

• swh/devel/swh-loader-mercurial!144 (merged): Fix implementation
• swh/devel/swh-loader-mercurial!144 (merged): Ensure no regression test
• #5261 (closed): Deploy
• Analysis