Because some of the JSON blocks in frontends.rst included non-JSON
contents (user replies, etc.) the blocks didn't parse/highlight and
`make html` complained:
borg/docs/internals/frontends.rst:28: WARNING: Could not lex literal_block as "json".
There's no easy way to enable and disable highlighting just for
specific lines, but individually unsetting the highlight language per
block suppresses the warnings.
borg list by default does not compute expensive values except when
they are needed. whether they are needed is determined by the format,
in standard mode as well as in --json mode.
When opening a repository, always try to read the magic number of the
latest segment and compare it to the Attic segment magic (unless the
repository is opened for upgrading). If an Attic segment is detected,
raise a dedicated exception, telling the user to upgrade the repository
first.
Fixes#1933.
