checkout: document bug where delayed checkout counts entries twice

checkout: document bug where delayed checkout counts entries twice

At the end of a `git checkout <pathspec>` operation, git reports how
many paths were checked out with a message like "Updated N paths from
the index". However, entries that end up on the delayed checkout queue
(as requested by a long-running process filter) get counted twice,
producing a wrong number in the final report. We will fix this bug in an
upcoming commit. For now, only document/demonstrate it with a
test_expect_failure.

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>