The following 3 patches implement this feature.

Internally the message object learns about flags.  Only a single flag is
defined to denote that a message matched the search expression.

That flag is then rendered on the "message{" line in the output of
notmuch show like this:

message{ id:... depth:4 match:0 filename:...
message{ id:... depth:4 match:1 filename:...

This can now be used by UI interfaces to hide or collapse less
interesting messages.

-Bart