Function: handle-shift-selection

Activate/deactivate mark depending on invocation thru shift translation.
This function is called by `call-interactively' when a command
with a `^' character in its `interactive' spec is invoked, before
running the command itself.

If `shift-select-mode' is enabled and the command was invoked
through shift translation, set the mark and activate the region
temporarily, unless it was already set in this way. See
`this-command-keys-shift-translated' for the meaning of shift

Otherwise, if the region has been activated temporarily,
deactivate it, and restore the variable `transient-mark-mode' to its earlier value.