bug#23397: 25.0.92; assertion failure auto-reverting a file being overwritten

[Eli Zaretskii]

> From: Noam Postavsky <npostavs@gmail.com>
> Cc: Eli Zaretskii <eliz@gnu.org>,  23397@debbugs.gnu.org
> Date: Sat, 16 Jun 2018 17:45:36 -0400
> 
> I don't quite understand what the overlap expression is trying to
> compute though.

It prevents us from re-using the same portion of buffer text for
matching both at start and at end of the file.  This could happen,
e.g., with buffers whose contents is the same character, and a file
whose contents is mostly the same, but larger, and with a few
different characters in the middle.  Then the "same_at_end" loop could
inadvertently match a part of the buffer that was already matched by
the "same_at_start" loop.

The overlap expression computes the portion of the buffer that was
matched by both "same_at_start" and "same_at_end" loops.