dir-iterator: support iteration in sorted order

dir-iterator: support iteration in sorted order

The `struct dir_iterator` is a helper that allows us to iterate through
directory entries. This iterator returns entries in the exact same order
as readdir(3P) does -- or in other words, it guarantees no specific
order at all.

This is about to become problematic as we are introducing a new reflog
subcommand to list reflogs. As the "files" backend uses the directory
iterator to enumerate reflogs, returning reflog names and exposing them
to the user would inherit the indeterministic ordering. Naturally, it
would make for a terrible user interface to show a list with no
discernible order.

While this could be handled at a higher level by the new subcommand
itself by collecting and ordering the reflogs, this would be inefficient
because we would first have to collect all reflogs before we can sort
them, which would introduce additional latency when there are many
reflogs.

Instead, introduce a new option into the directory iterator that asks
for its entries to be yielded in lexicographical order. If set, the
iterator will read all directory entries greedily and sort them before
we start to iterate over them.

While this will of course also incur overhead as we cannot yield the
directory entries immediately, it should at least be more efficient than
having to sort the complete list of reflogs as we only need to sort one
directory at a time.

This functionality will be used in a follow-up commit.

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