kbuild: give up untracked files for source package builds

kbuild: give up untracked files for source package builds

When the source tree is dirty and contains untracked files, package
builds may fail, for example, when a broken symlink exists, a file
path contains whitespaces, etc.

Since commit 05e96e96a315 ("kbuild: use git-archive for source package
creation"), the source tarball only contains committed files because
it is created by 'git archive'. scripts/package/gen-diff-patch tries
to address the diff from HEAD, but including untracked files by the
hand-crafted script introduces more complexity. I wrote a patch [1] to
make it work in most cases, but still wonder if this is what we should
aim for.

To simplify the code, this patch just gives up untracked files. Going
forward, it is your responsibility to do 'git add' for what you want in
the source package. The script shows a warning just in case you forgot
to do so. It should be checked only when building source packages.

[1]: https://lore.kernel.org/all/CAK7LNAShbZ56gSh9PrbLnBDYKnjtTkHMoCXeGrhcxMvqXGq9=g@mail.gmail.com/2-0001-kbuild-make-package-builds-more-robust.patch

Fixes: 05e96e96a315 ("kbuild: use git-archive for source package creation")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>