bug#15513: 24.2; isearch should clear selection in transient-mark-mode

[Drew Adams <drew.adams@oracle.com>]

> 3. Select the word This by pressing Ctrl+Shift+Right arrow.
> 4. Invoke isearch by pressing Ctrl+S.
> 5. Enter `notes' as the search string.
> 
> Observed behavior:
> * Selection is extended to include the word that was found by isearch.

Yes, that is the expected (designed, documented) behavior.  This was
discussed in help-gnu-emacs, in (your) thread "automatic selection
during search".

> Expected behavior:
> * Since isearch is a point movement command and such commands deactivate
> the selection in transient-mark-mode, I expect selection to be
> deactivated.

Isearch is a lot more than just a point movement command.  And it might
or might not actually move point, in the end.  And there is no rule that
I know of that says that all point-movement commands (meaning what? any
command that moves point?) must deactivate the region before it acts.
What about `S-<right>'?  That's a point movement command - should it
too deactivate the region?

> * If I press Ctrl+Space before searching, this indicates an explicit
> request for persistent selection. In this case, the current behavior of
> isearch is correct.

You activated the region by doing `C-S-<right>', indicating an explicit
request for an active region.  Isearch extends the active region.
If you don't want that, then don't activate the region (or else deactivate
it) before invoking Isearch.

IOW, if it hurts, just don't do that.