vc-clear-context should fill with 0, not nil

[Kalle Olavi Niemitalo]

In GNU Emacs 21.1.1 (i386-debian-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2001-10-29 on Astalo, modified by Debian
configured using `configure  i386-debian-linux-gnu --prefix=/usr 
--sharedstatedir=/var/lib --libexecdir=/usr/lib --localstatedir=/var/lib 
--infodir=/usr/share/info --mandir=/usr/share/man --with-pop=yes --with-x=yes 
--with-x-toolkit=athena'
Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: fi_FI
  locale-coding-system: iso-latin-1
  default-enable-multibyte-characters: t

The vc-clear-context function does:

  (fillarray vc-file-prop-obarray nil)

This is incorrect: unused elements of an obarray should contain
0, not nil, because "nil" can be interned.  (vc-hooks.el also
initializes the vector with zeros.)  So the form should be:

  (fillarray vc-file-prop-obarray 0)

In addition, the initialization in vc-hooks.el makes a vector of
length 16.  I don't have the Emacs 21 Lisp manual yet but the
Emacs 20.3 manual says the length of an obarray should be either
a prime or one less than a power of two.  Unless that has been
changed in Emacs 21, a length of 15 or 17 would be better.

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