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