From 7.4.135 to 7.4.182 now leaves backup files behind

I've been happily using 7.4.135 under Cygwin on Windows 7 since its
release. Today I installed (via Cygwin's own "setup.exe"
pseudo-package-manager) 7.4.182, and started running into problems.

Specifically, after editing files, I'm find Vim swap/backup/something
files left behind. They're binary files that start with "VIM", and
they all follow the same naming scheme: for file FOO, they're left
behind in ".FOO.un~".

Nothing in my .vimrc has changed. Both "backup" and "writebackup" are
off on the computer in question. I tried editing with -Vtest, but the
"test" file didn't have any particular error messages or other output
(lots and lots of chdirs....).

Reverting to 7.4.135 makes the spurious swap/backup/?? files go away.

I looked through ftp://ftp.vim.org/pub/vim/patches/7.4/README and none
of the entries between 135 and 182 jumped out at me as obvious
candidates for what's changed. I'm expecting that this is something I
can work around in my .vimrc, once I know what's causing it:

- what are the .un~ files?
- what vim settings control them?
- what changed in the latest patches to toggle the behavior?


Thank you for advice,
Ti

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