Some SWHIDs provided by the WebApp produce 404 when resolved

SWHIDs obtained via the WebApp are expected to be all resolvable, but I stumbled on a corner case that systematically fails.

To reproduce:

1. go to https://archive.softwareheritage.org/swh:1:rev:52f5f708b220b6a006fa64d54f915beb73cb65cf;origin=https://github.com/rdicosmo/parmap;visit=swh:1:snp:1f5f82e8e604e7aabc815c99bd0d15769e60016e/
2. pull out the Permalinks tab and click on "Directory", you get https://archive.softwareheritage.org/swh:1:dir:fc9727beb9a23027b7bcca391569ac81f4901c81;origin=https://github.com/rdicosmo/parmap;visit=swh:1:snp:1f5f82e8e604e7aabc815c99bd0d15769e60016e;anchor=swh:1:rev:52f5f708b220b6a006fa64d54f915beb73cb65cf/
3. this last link produces a 404: NotFoundExc: Branch refs/pull/97/head for snapshot with id 1f5f82e8e604e7aabc815c99bd0d15769e60016e not found!

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Migrated from T3195 (view on Phabricator)