fix 'list --json-lines' command when only a single result line is out…

[rblenis]

if only a single line of output comes from borg --json-lines, then the default processing will already have converted it to a dict, whereas if multiple lines are output, it leaves it as a string and lets the command specific code deal with it. This fixes the immediate problem of handling it in the 'list --json-lines' command, but will need to handle in the 'diff --json-lines' option when added to borg.

[m3nu]

I'll merge this next week, since the Debian guys asked me to do a feature freeze until Mar 10.

[m3nu]

Thanks for the addition! 🙏