bug#36859: Customizable fit-window-to-buffer

[martin rudalics]

> Before disabling window shrinking we need to support its
> customization, so shrinking could be enabled easily
> by anyone who wants it.

Doing this via 'display-buffer' is hardly feasible: We would have to
provide some magic that adds the (window-height) and (window-width)
functionalities to the sentinel of the process.  vc.el uses an advice
for that and I'm afraid that's one step too far for `display-buffer'.

The only practicable solution IMHO is to provide a plain
'vc-adjust-window-size-function' option defaulting to
'shrink-window-if-larger-than-buffer' and allowing 'ignore' and
'fit-window-to-buffer' as alternatives.

martin