only perform notmuch search if search term changes

[frasertweedale]

Describe the bug
If you edit the search, but don't actually change it, Purebred still performs the search.
The user incurs a wait to perform the search again,.

Expected behavior
No-op action when search term didn't actually change.

Additional context
There is a case where this behaviour may be desired: when new mail has appeared,
it may be necessary to execute the search again in order to see it. One possible
way to handle this is to augment the search syntax to allow user to say "do it anyway".
For example, a trailing exclamation point (!).