emacs: notmuch-search-refresh-view: reuse buffer

There's no reason to completely kill a buffer while refreshing its
search results because the buffer name is constant between refreshes
(based on the search query), only its contents may change and notmuch
search kills all local variables, so it's safe to reuse.

Reusing the same buffer also makes it possible to do things like
refreshing a buffer which is not focused or even not shown in any
window - this will be used in the next commits to add auto-refresh
capabilities to all existing notmuch buffers + a function to call
after syncing mail to refresh everything.

Signed-off-by: Ioan-Adrian Ratiu <adi@adirat.com>