git-arc: Make patch with reviewers more portable

[jlduran_gmail.com]

Summary

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.

[markj]

I see a couple of other uses of echo -n, in gitarc__list().

[jlduran_gmail.com]

Fix other occurrences of "echo -n" while here.