git-arc: fix diff2reviewers() with multiple reviewers

The select command of jq provides us with a multiline output. Thus, the
${userids} is a newline separated list. The recent change 9e84289f2c7e
explicitly said that we aren't going to translate newline to comma when
constructing JSON for the next command. It also forces the ${userids} to
be printed as is, with shell's field splitting disabled. So we end up
with invalid JSON, that is missing commas.

Fix this by enabling shell's field splitting by printing ${userids}
outside of double quotes.

Reviewed by: jlduran, imp, emaste
Differential Revision: https://reviews.freebsd.org/D47889
Fixes: 9e84289f2c7e6b632047a75b653fcd587287a8b9