When opening a Find or Replace prompt, prefill the input with
the most recent search pattern from the history. The user can
still type freely to override or press Backspace to clear it.
Only applies to Find / Replace prompts (not GotoLine, etc.).
Updated the existing alt_slash test to account for the
auto-suggested prefix (it now presses Backspace 5 times to
clear 'hello' before typing 'world').
Tests: 1407 pass, zero warnings.
vasilito
ef3f696689