Set up the post global variable in the comment feed loops so that any calls to...

Set up the post global variable in the comment feed loops so that any calls to post related template tags work correctly.
Ensures that atom feeds show threading for top-level comments correctly.
Fixes #14908 props solarissmoke and SergeyBiryukov.

git-svn-id: https://core.svn.wordpress.org/trunk@18716 1a063a9b-81f0-0310-95a4-ce76da25c4cd