git-arc: Make patch with reviewers more portable

Avoid an ArcanistUsageException:

Provide method parameters on stdin as a JSON blob.

Due to an invalid JSON input on strict POSIX-compliant shells (macOS):

echo '{
        "constraints": {"phids": [-n,"PHID-USER-xxx",]}
      }'                          ^^^               ^

Change the non-portable "echo -n" to a printf.

Reviewed by: 0mp, imp, markj
Approved by: markj (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D46781