wt-status: fix leaking buffer with sparse directories

wt-status: fix leaking buffer with sparse directories

When hitting a sparse directory in `wt_status_collect_changes_initial()`
we use a `struct strbuf` to assemble the directory's name. We never free
that buffer though, causing a memory leak.

Fix the leak by releasing the buffer. While at it, move the buffer
outside of the loop and reset it to save on some wasteful allocations.

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