cygdrive prefix /mnt and vim swap files

[A Higgs <asherhiggs at gmail dot com>]

When the cygdrive prefix is /mnt (via fstab modification) vim swap
file creation malfunctions as following. Shell working directory is ~
in the examples but doesn't seem to matter:

vi -p foo /mnt/d/bar
(swap files created for both)
vi -p /mnt/d/bar foo
(swap file only created for bar)
vi -p /mnt/d/foo /mnt/d/bar
(swap file only created for foo)
vi -p foo bar
(swap files created for both)
vi -p foo /mnt/d/bar bar
(swap files created for foo and /mnt/d/bar)

Basically, it appears that after opening the first file on a cygdrive
no further swap files are created. Opening files in windows rather
than tabs, or opening additional files from within vim (eg. with
tabedit) makes no difference. Nor does the particular cygdrive or its
filesystem (eg. /mnt/e/foo where E: is FAT). I have also ruled out vim
configuration as the cause by removing ~/.vimrc & ~/.vim/ .

However, this behaviour doesn't occur when the cygdrive prefix is
reset to /cygdrive (or set to /foo or /mtn). I don't have a 'mnt'
directory in my cygwin install directory, and otherwise I have no idea
what to make of this. I only discovered the issue by accident and am
not particularly concerned with a solution or workaround, but I'm
prepared to offer more information or troubleshooting if necessary.
Thanks.

Windows 7 Professional Ver 6.1 Build 7601 Service Pack 1
CYGWIN_NT-6.1 2.5.2(0.297/5/3) 2016-06-23 14:29 x86_64
vim 7.4.1990-1

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