[PATCH] Fix for bug #19474: LIKE fails to match literal backslashes with nondeterministic collations

It was reported in [1] that LIKE on a non-deterministic collation
returns an incorrect result when the pattern contains a literal
backslash.

This was caused by escaping all backslashes in like_match.c (even when
the internal pattern contained '\\'). This behaviour has been present
since 85b7efa1cd which originally added support for LIKE with
non-deterministic collations.

This patch fixes the issue by always including the character after a
literal '\' in the final buffer. I didn't check for the end of the
string because that check is already handled in the block above when
checking for escape characters.

I also added a regression test for this issue and confirmed that it
passes with the fix.

Please take a look and let me know what you folks think.

[1] https://www.postgresql.org/message-id/flat/19474-5b86a95f3d9a7ecb%40postgresql.org

Thanks & Regards,
Nitin Motiani
Google