emacs: Refresh hello whenever the user switches to the buffer

emacs: Refresh hello whenever the user switches to the buffer

Previously, we refreshed hello when the user quit a search that was
started from hello.  This is fine assuming purely stack-oriented
buffer use, but is quite fragile and requires hacks to search.

This replaces that logic with a new approach that refreshes hello
whenever the user switches to the hello buffer, regardless of how this
happens.