grep: fix leak in `grep_splice_or()`

grep: fix leak in `grep_splice_or()`

In `grep_splice_or()` we search for the next `TRUE` node in our tree of
grep expressions and replace it with the given new expression. But we
don't free the old node, which causes a memory leak. Plug it.

This leak is exposed by t7810, but plugging it alone isn't sufficient to
make the test suite pass.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>