FUSE: fix various paper cuts (user testing 2020-11-24)

Paper cuts identified during the user testing session held on 2020-11-24 (participants: @haltode, @seirl, @zack):

ENOENT shouldn't be logged (#2802 (closed))
Use yaml.dump() to display the config (only in debug instead of info) (!39 (closed))
Document how to change the logging level (!40 (closed))
DEBUG level: only return the number of results, to have a readable output (→ general solution for swh CLI: swh-core#2819 (closed))
pretty-print the meta JSON (!41 (closed))
try to find why each zsh prompt (triggering plugins) is so slow (maybe lookup() → O(1) results in the sqlite database?)
update .status every time it is opened, not only on ls (!54 (closed))
nested branch names (e.g., feature/foo) in snapshot instead of %2F (#2820 (closed))
origin/archive search -> swh search CLI, returning origins (T2822)

Migrated from T2811 (view on Phabricator)