FUSE: nest branch names in snapshot views instead of URL-escaping slashes

i.e., we want to switch from the current flat view of branches:

swhfs/archive/swh:1:snp:2ca5d6eff8f04a671c0d5b13646cede522c64b7d $ ls | head -n 5
HEAD
refs%2Fheads%2F386BSD-0.0-Snapshot-Development
refs%2Fheads%2F386BSD-0.1-patchkit
refs%2Fheads%2F386BSD-0.1-patchkit-Import
refs%2Fheads%2F386BSD-0.1-Snapshot-Development

to nested sub-directories following the "/" present in branch names, like this:

HEAD
refs/heads/386BSD-0.0-Snapshot-Development
refs/heads/386BSD-0.1-patchkit
refs/heads/386BSD-0.1-patchkit-Import
refs/heads/386BSD-0.1-Snapshot-Development

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Migrated from T2820 (view on Phabricator)