archive: don't add empty directories to archives

While git doesn't track empty directories, git archive can be tricked
into putting some into archives.  One way is to construct an empty tree
object, as t5004 does.  While that is supported by the object database,
it can't be represented in the index and thus it's unlikely to occur in
the wild.

Another way is using the literal name of a directory in an exclude
pathspec -- its contents are are excluded, but the directory stub is
included.  That's inconsistent: exclude pathspecs containing wildcards
don't leave empty directories in the archive.

Yet another way is have a few levels of nested subdirectories (e.g.
d1/d2/d3/file1) and ignoring the entries at the leaves (e.g. file1).
The directories with the ignored content are ignored as well (e.g. d3),
but their empty parents are included (e.g. d2).

As empty directories are not supported by git, they should also not be
written into archives.  If an empty directory is really needed then it
can be tracked and archived by placing an empty .gitignore file in it.

There already is a mechanism in place for suppressing empty directories.
When read_tree_recursive() encounters a directory excluded by a pathspec
then it enters it anyway because it might contain included entries.  It
calls the callback function before it is able to decide if the directory
is actually needed.  For that reason git archive adds directories to a
queue and writes entries for them only when it encounters the first
child item -- but currently only if pathspecs with wildcards are used.

Queue *all* directories, no matter if there even are pathspecs present.
This prevents git archive from writing entries for empty directories in
all cases.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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