git-arc: Prefer echo over printf

Prefer echo over printf.  Prevent the newline character from being
translated by using 'blank' instead of 'space' in the re_format.

This change was reviewed in:

https://reviews.freebsd.org/D47289

Committed separately for clarity.

Suggested by:	des
Approved by:	emaste (mentor)

tools/tools/git/git-arc.sh

@@ -341,7 +341,7 @@ diff2reviewers()
         jq '.response.data[0].attachments.reviewers.reviewers[] | select(.status == "accepted").reviewerPHID')
     if [ -n "$userids" ]; then
         echo '{
-        "constraints": {"phids": ['"$(printf "%s" "$userids" | tr '[:space:]' ',')"']}
+        "constraints": {"phids": ['"$(echo "$userids" | tr '[:blank:]' ',')"']}
         }' |
         arc_call_conduit -- user.search |
         jq -r '.response.data[].fields.username'