bug#73232: [PATCH] Allow vc-diff to suggest a default revision in vc-dir

[Dmitry Gutov <dmitry@gutov.dev>]

Hi all,

On 14/09/2024 04:45, Dmitry Gutov wrote:
> 
> I can see where you're coming from though -- that default isn't very 
> useful, one might as well not press C-u.
> 
> Maybe we should switch to suggesting the previous revision in the prompt 
> even when file has changes?

Here's what seems to me an overall improvement, based on the original 
change. And more consistent as well.

* No special case for when FIRST is a directory OR it's not up-to-date.
* Make REV1-DEFAULT a list value.
* In 'vc-root-version-diff', don't try calling 'vc-deduce-fileset' and 
construct a (BACKEND DEFAULT-DIR) fileset right away.

As a result, 'C-u C-x v d' consistently provides completion and diff 
relating to the whole repository, not for files as point (if any). 
Previously, it used the revision that last touched the corresponding 
file, or nil, if the file was untracked (e.g. in Dired).

Further, don't offer the working revision as REV1-DEFAULT. Except for 
historical reasons and some idea of consistency, I can't see a scenario 
where that would be useful, which would not be covered by calling 'C-x v 
d' without a prefix. Someone please correct me here.

And combined with Spencer's patch from https://debbugs.gnu.org/62940#46, 
we get this:

* First default is HEAD^ (the last revision before the latest).
* Second default is @{upstream}.
* Then the elements from vc-revision-history.

WDYT?