bug#41821: 28.0.50; read-directory-name in vc commands should provide defaults from projects

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 41821@debbugs.gnu.org, juri@linkov.net
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Thu, 25 Jun 2020 16:50:24 +0300
> 
> > In general, code that doesn't _have_ to be preloaded, shouldn't be.
> > If nothing else, it keeps the memory footprint of a bare Emacs
> > smaller, and thus prevents us from slipping down the slippery slope of
> > memory bloat.
> 
> And having vc-hooks call project.el functions at runtime would somehow 
> force us to preload it? How?

It's enough that someone makes the function a macro, or does something
else similarly innocent.

> > Anyway, if you-two feel strongly about keeping the current solution,
> > i.e. having VC commands use project.el-collected history, I'd
> > appreciate if that function could be moved to vc.el from vc-hooks.el,
> > thanks in advance.
> 
> We can't just move it: it accesses information private to project.el. 
> The best we could do is a wrapper function.

Sorry, I don't understand: why can't we move it from vc-hooks.el to
vc.el, and why doing that would need a wrapper?