git-arc: Do not echo unescaped literals to jq

Store the json blobs in temporary files instead of variables, and feed
them to jq for parsing.

When echoing "\n", the new line will become a literal new line, and
therefore, invalid json input:

```
jq: parse error: Invalid string: control characters from U+0000 through U+001F must be escaped ...
```

To reproduce:

    % git arc patch -c D12345

Reviewed by:	0mp, imp, markj
Approved by:	markj (mentor)
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D46767

tools/tools/git/git-arc.sh

@@ -548,32 +548,32 @@ patch_commit()
     diff=$1
     reviewid=$(diff2phid "$diff")
     # Get the author phid for this patch
-    review_data=$(echo '{
-                  "constraints": {"phids": ["'"$reviewid"'"]}
-		}' |
-        arc_call_conduit -- differential.revision.search)
-    authorid=$(echo "$review_data" | jq -r '.response.data[].fields.authorPHID' )
+    review_data=$(mktemp)
+    echo '{"constraints": {"phids": ["'"$reviewid"'"]}}' | \
+        arc_call_conduit -- differential.revision.search > "$review_data"
+    authorid=$(jq -r '.response.data[].fields.authorPHID' "$review_data")
     # Get metadata about the user that submitted this patch
-    user_data=$(echo '{
-                  "constraints": {"phids": ["'"$authorid"'"]}
-		}' |
-            arc call-conduit -- user.search | grep -v ^Warning: |
-            jq -r '.response.data[].fields')
-    user_addr=$(echo "$user_data" | jq -r '.username')
-    user_name=$(echo "$user_data" | jq -r '.realName')
+    user_data=$(mktemp)
+    echo '{"constraints": {"phids": ["'"$authorid"'"]}}' | \
+        arc_call_conduit -- user.search | \
+        jq -r '.response.data[].fields' > "$user_data"
+    user_addr=$(jq -r '.username' "$user_data")
+    user_name=$(jq -r '.realName' "$user_data")
+    rm "$user_data"
     # Dig the data out of querydiffs api endpoint, although it's deprecated,
     # since it's one of the few places we can get email addresses. It's unclear
     # if we can expect multiple difference ones of these. Some records don't
     # have this data, so we remove all the 'null's. We sort the results and
     # remove duplicates 'just to be sure' since we've not seen multiple
     # records that match.
-    diff_data=$(echo '{
-		"revisionIDs": [ '"${diff#D}"' ]
-		}' | arc_call_conduit -- differential.querydiffs |
-	     jq -r '.response | flatten | .[]')
-    author_addr=$(echo "$diff_data" | jq -r ".authorEmail?" | sort -u)
-    author_name=$(echo "$diff_data" | jq -r ".authorName?" | sort -u)
+    diff_data=$(mktemp)
+    echo '{"revisionIDs": [ '"${diff#D}"' ]}' | \
+        arc_call_conduit -- differential.querydiffs |
+        jq -r '.response | flatten | .[]' > "$diff_data"
+    author_addr=$(jq -r ".authorEmail?" "$diff_data" | sort -u)
+    author_name=$(jq -r ".authorName?" "$diff_data" | sort -u)
     author=$(find_author "$user_addr" "$user_name" "$author_addr" "$author_name")
+    rm "$diff_data"
 
     # If we had to guess, and the user didn't want to guess, abort
     if [ "${author}" = "ABORT" ]; then
@@ -582,10 +582,11 @@ patch_commit()
     fi
 
     tmp=$(mktemp)
-    echo "$review_data" | jq -r '.response.data[].fields.title' > $tmp
+    jq -r '.response.data[].fields.title' "$review_data" > $tmp
     echo >> $tmp
-    echo "$review_data" | jq -r '.response.data[].fields.summary' >> $tmp
+    jq -r '.response.data[].fields.summary' "$review_data" >> $tmp
     echo >> $tmp
+    rm "$review_data"
     # XXX this leaves an extra newline in some cases.
     reviewers=$(diff2reviewers "$diff" | sed '/^$/d' | paste -sd ',' - | sed 's/,/, /g')
     if [ -n "$reviewers" ]; then