C-x v u via *.~REV.~ reset file date to 1970-01-01

[Simon Josefsson <jas@extundo.com>]

Visit a file in CVS and type a SPC and then remove it and save the
file. VC generate a backup file dated 1970-01-01:

jas@latte:~/src/libidn$ ls -la Makefile.am*
-rw-r--r--    1 jas      staff        1959 2003-08-26 04:22 Makefile.am
-rw-r--r--    1 jas      staff        1959 1970-01-01 01:00 Makefile.am.~1.91.~
jas@latte:~/src/libidn$ 

If you then do C-x v u, Emacs move the backup file back to the
original, but keeps the file date.  BSD make seem to use time=0 to
mean that the file doesn't exist, so you get an error that the file
doesn't exist, which is rather confusing.

For CVS, why not use the file date stored in CVS/Entries?  It would be
even better than using current time, which otherwise would be one
solution.