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

[Dmitry Gutov]

Hi Lars,

On 10.10.2019 1:40, Lars Ingebrigtsen wrote:
> But we see that it's used, and used for one thing only: If we call this
> function in a dired buffer, the function will signal an error if
> OBSERVER is nil.  So all the callers of this function pass in t if it's
> a non-state-changing operation.

This is my impression as well.

> So the parameter does make some sense: OBSERVER non-nil means that it's
> a non-destructive operation we're doing to do (but we only care in dired
> buffers).

Exactly. So step 1 is probably to document is with 1-2 lines in a 
docstring. Maybe a rename as well? E.g. to READONLY-OPERATION.

> Having that check in this function may be a bit odd, but...

I think it's fine to have it there, since most VC commands end up 
calling this function.

What I don't fully understand, however, is why prohibit state-changing 
operations in Dired buffers. Is it just because there's no VC-Dir buffer 
easily at hand, to update the visible file statuses?

Or maybe it's for the user not to circumvent the "similar VC states" 
logic in VC-Dir.