Fix working directory errno handling when unlinking a directory

Fix working directory errno handling when unlinking a directory

Alex Riesen noticed that the case where a file replaced a directory entry
in the working tree was broken on cygwin. It turns out that the code made
some Linux-specific assumptions, and also ignored errors entirely for the
case where the entry was a symlink rather than a file.

This cleans it up by separating out the common case into a function of its
own, so that both regular files and symlinks can share it, and by making
the error handling more obvious (and not depend on any Linux-specific
behaviour).

Acked-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>