Test 2 of t5004 checks if a supposedly empty tar archive really
contains no files.  24676f02 (t5004: fix issue with empty archive test
and bsdtar) removed our commit hash to make it work with bsdtar, but
the test still fails on NetBSD and OpenBSD, which use their own tar
that considers a tar file containing only NULs as broken.

Here's what the different archivers do when asked to create a tar
file without entries:

	$ uname -v
	NetBSD 6.0.1 (GENERIC)
	$ gtar --version | head -1
	tar (GNU tar) 1.26
	$ bsdtar --version
	bsdtar 2.8.4 - libarchive 2.8.4

	$ : >zero.tar
	$ perl -e 'print "\0" x 10240' >tenk.tar
	$ sha1 zero.tar tenk.tar
	SHA1 (zero.tar) = da39a3ee5e6b4b0d3255bfef95601890afd80709
	SHA1 (tenk.tar) = 34e163be8e43c5631d8b92e9c43ab0bf0fa62b9c

	$ : | tar cf - -T - | sha1
	da39a3ee5e6b4b0d3255bfef95601890afd80709
	$ : | gtar cf - -T - | sha1
	34e163be8e43c5631d8b92e9c43ab0bf0fa62b9c
	$ : | bsdtar cf - -T - | sha1
	34e163be8e43c5631d8b92e9c43ab0bf0fa62b9c

So NetBSD's native tar creates an empty file, while GNU tar and bsdtar
both give us 10KB of NULs -- just like git archive with an empty tree.
Now let's see how the archivers handle these two kinds of empty tar
files:

	$ tar tf zero.tar; echo $?
	tar: Unexpected EOF on archive file
	1
	$ gtar tf zero.tar; echo $?
	gtar: This does not look like a tar archive
	gtar: Exiting with failure status due to previous errors
	2
	$ bsdtar tf zero.tar; echo $?
	0

	$ tar tf tenk.tar; echo $?
	tar: Cannot identify format. Searching...
	tar: End of archive volume 1 reached
	tar: Sorry, unable to determine archive format.
	1
	$ gtar tf tenk.tar; echo $?
	0
	$ bsdtar tf tenk.tar; echo $?
	0

NetBSD's tar complains about both, bsdtar happily accepts any of them
and GNU tar doesn't like zero-length archive files.  So the safest
course of action is to stay with our block-of-NULs format which is
compatible with GNU tar and bsdtar, as we can't make NetBSD's native
tar happy anyway.

We can simplify our test, however, by taking tar out of the picture.
Instead of extracting the archive and checking for the non-presence of
files, check if the file has a size of 10KB and contains only NULs.
This makes t5004 pass on NetBSD and OpenBSD.
Signed-off-by: NRene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Versions of tar that don't know pax headers -- like the ones in NetBSD 6
and OpenBSD 5.2 -- extract them as regular files.  Explicitly ignore the
file created for our global header when checking the list of extracted
files, as this is normal and harmless fall-back behaviour.  This fixes
test 3 of t5004 on these platforms.
Signed-off-by: NRene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

bsdtar, which is the default tar on Mac OS X, handles empty archives
just fine but reports archives containing only a pax extended header
comment as damaged.  Work around the issue by explicitly generating
the archive for the tree and not the commit, which causes git archive
to omit the commit hash comment record from the tar file.
Reported-by: NBJ Hargrave <bj@bjhargrave.com>
Signed-off-by: NRene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-archive relies on get_pathspec to convert its argv into
a list of pathspecs. When get_pathspec is given an empty
argv list, it returns a single pathspec, the empty string,
to indicate that everything matches. When we feed this to
our path_exists function, we typically see that the pathspec
turns up at least one item in the tree, and we are happy.

But when our tree is empty, we erroneously think it is
because the pathspec is too limited, when in fact it is
simply that there is nothing to be found in the tree. This
is a weird corner case, but the correct behavior is almost
certainly to produce an empty archive, not to exit with an
error.

This patch teaches git-archive to create empty archives when
there is no pathspec given (we continue to complain if a
pathspec is given, since it obviously is not matched). It
also confirms that the tar and zip writers produce sane
output in this instance.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>