Comments: Add `In reply to %s.` prefix to child comments in comments feed.

The prefix includes a link to the parent comment and the name of its author.

Props zodiac1978, imath, SergeyBiryukov.
Fixes #43429.
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git-svn-id: https://core.svn.wordpress.org/trunk@46911 1a063a9b-81f0-0310-95a4-ce76da25c4cd