bug#52896: 29.0.50; transient-mark-mode is set to a lot of 'only syms

[Andrey Listopadov <andreyorst@gmail.com>]

> How come transient-mark-mode became set to 'only' in the first place?
> That is supposed to happen only when shift-translated commands are
> used, and should be reset to the old value whenever the first
> non-shift-translated command is used.  See the doc string of
> transient-mark-mode (the variable).

The expand-region package sets transient mark mode to `only' when tmm
is disabled:

https://github.com/magnars/expand-region.el/blob/4b8322774d9c1d8b64a0049d1dbbc1e7ce80c1a0/expand-region-core.el#L57
https://github.com/magnars/expand-region.el/blob/4b8322774d9c1d8b64a0049d1dbbc1e7ce80c1a0/expand-region-core.el#L163

> You can see how it works if, in "emacs -Q", you press and hold Shift
> and use arrow keys to move point, then evaluate
>
>   M-: transient-mark-mode RET
>
> You should then see the value '(only . t)' of the variable.  Once you
> type some unshifted command, like C-f, the value becomes t again.

Yes, I know, but in the case when transient mark mode is disabled, its
value is not `(only . t)', but `(only . nil)' which is just `(only)'.
And since expand-region conses `only' initially, the
`exchange-point-and-mark' function then rapidly conses a lot of
symbols to it as well, since expand region uses it.

> So I suspect that the expand-region.el package you installed does
> something to cause this, and in particular doesn't take care to reset
> the value when it should.

I don't think it should reset it, e.g. it behaves well, and sets it to
allow shift-select to work:

https://github.com/magnars/expand-region.el/pull/225

> Bottom line: you should look in expand-region.el for the reasons of
> this misbehavior, and perhaps report a bug to its developers.

Unfortunately, Magnar Sveen disabled the issue-reporting feature of
github for this particular project.
However, looking at the docstring, I guess what expand-region does is
correct, and the `exchange-point-and-mark' function does kind of a
weird check - e.g. it checks if the car of `transient-mark-mode' is
`only' and then conses another only to that.
But again, I'm not sure what's the intention here, maybe it is expected.

-- 
Andrey Listopadov