debian/rules: don't call $(MAKE) -Ccontrib/mw-to-git without $(OPTS)

debian/rules: don't call $(MAKE) -Ccontrib/mw-to-git without $(OPTS)

Since debian/rules calls $(MAKE) -Ccontrib/mw-to-git all without $(OPTS),
the root Makefile detects that the prefix has changed from /usr to $HOME,
and winds up installing Git.pm to the wrong directory.

  /usr/bin/make -C perl prefix='/usr' DESTDIR='/build/buildd/git-1.8.5~rc2/debian/git' install
  make[2]: Entering directory `/build/buildd/git-1.8.5~rc2/perl'
  make[3]: Entering directory `/build/buildd/git-1.8.5~rc2/perl'
  Installing /build/buildd/git-1.8.5~rc2/debian/git/home/buildd/share/perl5/Git.pm
[...]
  chmod 644 '/build/buildd/git-1.8.5~rc2/debian/git'/usr/share/perl5/Git.pm
  chmod: cannot access '/build/buildd/git-1.8.5~rc2/debian/git/usr/share/perl5/Git.pm': No such file or directory
  make: *** [install-arch] Error 1

This patch fixes it.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>