git status: Fix false positive "new commits" output for dirty submodules

git status: Fix false positive "new commits" output for dirty submodules

Testing if the output "new commits" should appear in the long format of
"git status" is done by comparing the hashes of the diffpair. This always
resulted in printing "new commits" for submodules that contained untracked
or modified content, even if they did not contain new commits. The reason
was that match_stat_with_submodule() did set the "changed" flag for dirty
submodules, resulting in two->sha1 being set to the null_sha1 at the call
sites, which indicates that new commits are present. This is changed so
that when no new commits are present, the same object names are in the
sha1 field for both sides of the filepair, and the working tree side will
have the "dirty_submodule" flag set when appropriate. For a submodule to
be seen as modified even when it just has a dirty work tree, some
conditions had to be extended to also check for the "dirty_submodule"
flag.

Unfortunately the test case that should have found this bug had been
changed incorrectly too. It is fixed and extended to test for other
combinations too.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>