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

[Spencer Baugh via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Dmitry Gutov <dmitry@gutov.dev> writes:
> And here's the diff.
>
> On 28/09/2024 00:49, Dmitry Gutov wrote:
>> 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?

This seems reasonable to me.  Then we have the following reliable
behaviors:

- [some diff command] RET
   diffs the working tree against HEAD
- C-u [some diff command] RET RET
   diffs the working tree against HEAD^
- C-u [some diff command] M-n RET RET
   diffs the working tree against @{upstream}

This seems like a big improvement, since previously would need to
actually read the prompt in the C-u case to figure out whether the
default was correct/what you wanted.