store: do not drop the final `/` when creating manifestlog instance

This bug, inherited from the upgrade code leads to the acces/creation of broken
revlog with name `DIRECTORY00manifest.i` instead of `DIRECTORY/00manifest.i`

We fix it in its own changeset to preserve the "pure code movement" aspect of
the previous changesets.

mercurial/store.py

@@ -554,7 +554,7 b' class RevlogStoreEntry(BaseStoreEntry):'
         if self.is_changelog:
             return changelog.changelog(repo.svfs)
         elif self.is_manifestlog:
-            mandir = self.target_id.rstrip(b'/')
+            mandir = self.target_id
             return manifest.manifestrevlog(
                 repo.nodeconstants, repo.svfs, tree=mandir
             )