Fix what to do and how to detect when hardlinking fails

Fix what to do and how to detect when hardlinking fails

Recent FAT workaround caused compilation trouble on OpenBSD;
different platforms use different error codes when we try to
hardlink the temporary file to its final location.  Existing
Coda hack also checks its own error code, but the thing is,
the case we care about is if link failed for a reason other
than that the final file has already existed (which would be
normal, or it could mean collision).  So just check the error
code against EEXIST.

Signed-off-by: Junio C Hamano <junkio@cox.net>