vc-previous-version makes unwarranted assumptions about revision numbers

[Jonathan Kamens]

In GNU Emacs 21.1.1 (i386-redhat-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2001-12-06 on stripples.devel.redhat.com

The function vc-previous-version assumes that revision numbers always
contain two or more numbers separated by periods.  This isn't true for
some SCM systems.  E.g., in Perforce, a file revision is a single
number -- no periods.

At the very least, vc-previous-version needs to be modified to know
about revision numbers that are a single number (in which case the
previous version is simply that number minus one).  Better would be
for a new SCM hook, vc-BACKEND-previous-version, to be added, so that
knowledge about revisions can be abstracted out of vc.el for some
SCMs.

  jik