R1:36075f6ce525

Correct a prose diff behavior when prose pieces include newlines

Summary:
See <https://discourse.phabricator-community.org/t/bad-regex-in-prose-diff-logic/3969>.

The prose splitting rules normally guarantee that newlines appear only at the beginning or end of blocks. However, if a prose sentence ends with text like "...x\n.", we can end up with a newline inside a "sentence".

If we do, the regular expression that breaks it into pieces will fail.

Arguably, this is an error in how sentences are split…