matchtree: Re-use regexpMatchTree.found
matchtree: Re-use regexpMatchTree.found

The code is already setup to reuse t.found, it just wasn't. We could make this
code a bit more complicated and re-allocate if cap(found) < len(idxs). But I
think this is unneccessary given we will reuse the slice now.

Change-Id: I47700abf6078c23b8cef6f23f25db01b84a21f26

google/zoekt,google/zoekt