VC now supports CVS per-file branches

[Andre Spiegel]

Thanks to Martin Lorentzson, VC now supports per-file branches under
CVS, in exactly the same way as under RCS.  The only difference is
that branches need to have symbolic names under CVS; these symbolic
names are now also displayed in the mode line.

I thought I'd post this announcement here, because it should make it
easier for everybody to work both on the trunk and the branch in the
Emacs sources.  I will update the manual as soon as I'm convinced
people have accepted the idea :-)

Below is Martin's comment on his patch.  Thanks Martin!

  - C-u C-x v v for a modified file: If the input revision is a valid
    symbolic tag name, `vc-cvs-checkin' create it as a branch, commit
    and switch to it.  It works as described in Chapter "Advanced
    Control in `C-x C-q'".  Chapter "Creating New Branches" needs
    updating (it only mentions version numbers, not symbolic names.)

    C-u C-x v v for a un-modified file: If the input revision is a
    valid symbolic tag name, we switch to it.  This was existing
    behaviour!  I have made no changes for this!

  - The display of the sticky tag in the mode-line is really a
    self-sustained, stand-alone addition.  It will display the CVS
    sticky tag, if any.  The sticky tag can be any of the following:
   
      1. A symbolic tag name.
      2. A symbolic tag name as a branch.
      3. A date.
      4. A revision number.

    The default display (if `vc-cvs-sticky-tag-display' is t) is to
    format case 3 using `vc-cvs-sticky-date-format-string' while case
    1, 2 and 4 are simply printed.

    There is no way (that I can think of) to separate case 1 and 2.
    Case 1/2, 3 and 4 are separated in the special function value
    parameter TYPE of variable `vc-cvs-sticky-tag-display'.

    The reason to give the user this amount of control is to provide
    the ability to save precious mode-line space.  The second Lisp
    example in the `vc-cvs-sticky-tag-display' documentation shows a
    good example of this.

    The sticky tag is displayed within parenthasis.  [I changed this
    to square brackets, AS.]  I couldn't find
    any guidelines on how to correctly display such information.
    Perhaps it is better displayed more closely to the existing CVS
    information?  Perhaps the parenthasis are redundant?

    Chapter "Version Control and the Mode Line" needs updating, should
    you decide to keep the sticky tag display.  (I think you should,
    it's seriously cool ;)