bug#34949: 27.0.50; Docstring of `vc-deduce-fileset' incomplete

[Juri Linkov]

>> So I implemented this in vc-dired-deduce-fileset whose basic
>> algorithm was taken from vc-dir-deduce-fileset.
>> Now with this patch state changing VC operations are supported on marked
>> _files_ in dired.
>
> TBH, I'm not sure this is a win (we have VC-Dir for a reason),

Often I need to quickly commit one or a few marked files in the same
Dired directory without needlessly starting VC-Dir - this is when such
feature comes handy.  Even it's possible to mark files in several
subdirectories inserted in the same Dired buffer.

> but if you really want to be able to do that in Dired, why not. Please
> feel free to install this.

Installed to master.

>> However, the need for the OBSERVER arg still remains because this patch
>> doesn't support state changing VC operations on marked _directories_ in 
>> dired.
>> It raises the error in this case:
>>    State changing VC operations on directories not supported in
>> `dired-mode'
>> State changing VC operations could be supported on directories as well,
>> but there are several possibilities to choose from, and I can't decide:
>> 1. On a marked directory get a state from all its files
>>     (probably this variant makes no sense)
>> 2. On a marked directory get only files with a "modified" state
>>     (i.e. edited/added/removed)
>
> You can check out what diff-hl-dired does in this regard (and it does use
> the dir-status-files command), though you would need to do something else
> than its merging logic.

Thanks, I use diff-hl-dired, but not realized it relies on dir-status-files.
Maybe dir-status-files could be simplified, so that it wouldn't require
a special buffer to be current, and could be called in any buffer,
but this simplification is not much needed.

> That still leaves the downside (compared to VC-Dir) of being unable to
> select individual files in subdirectories, but that's not fixable, I guess.

When calling the already supported ‘C-x v =’ (vc-diff) on a subdirectory
in Dired, it operates on all modified files, so it makes sense to commit
exactly the same files whose diffs are displayed in the *vc-diff* buffer.

The same way we could add a new command to commit all modified files
in the repository.  When ‘C-x v D’ (vc-root-diff) displays the
*vc-diff* buffer with all modifications, a new command like ‘C-x v V’
could commit all of them.

>> But the latter is not easy to implement because the needed functionality
>> is missing in VC, because the only VC method that is suitable for this is
>> 'dir-status-files' to get VC files from a directory but it's too tightly
>> integrated with vc-dir and can't be easily separated, i.e. there is
>> no easy way to do something like:
>>    (with-temp-buffer
>>      (vc-call-backend
>>       (vc-responsible-backend default-directory) 'dir-status-files 
>> default-directory nil
>>       (lambda (entries &optional more-to-come)
>>         (message "! %S" entries))))
>
> I think the only possible problem here is that lambda is invoked
> asynchronously. But we can sleep while waiting for it.

Would it be possible to call dir-status-files synchronously?