git-arc: Fix fetching of reviewers

[markj]

Summary

Let the shell normalize whitespace (i.e., convert newlines to spaces).
Otherwise we end up generating invalid JSON.

Fixes: 9e84289f2c7e ("git-arc: Prefer echo over printf")

[jlduran]

Is this the same as D47889?

[markj]

In D47893#1092074, @jlduran wrote:

Is this the same as D47889?

Yep, Gleb beat me to it. :)