kbuild: make W=1 warn files that are tracked but ignored by git

The top .gitignore comments about how to detect files breaking
.gitignore rules, but people rarely care about it.

Add a new W=1 warning to detect files that are tracked but ignored by
git. If git is not installed or the source tree is not tracked by git
at all, this script does not print anything.

Running it on v6.2-rc1 detected the following:

  $ make W=1 misc-check
  Documentation/devicetree/bindings/.yamllint: warning: ignored by one of the .gitignore files
  drivers/clk/.kunitconfig: warning: ignored by one of the .gitignore files
  drivers/gpu/drm/tests/.kunitconfig: warning: ignored by one of the .gitignore files
  drivers/hid/.kunitconfig: warning: ignored by one of the .gitignore files
  fs/ext4/.kunitconfig: warning: ignored by one of the .gitignore files
  fs/fat/.kunitconfig: warning: ignored by one of the .gitignore files
  kernel/kcsan/.kunitconfig: warning: ignored by one of the .gitignore files
  lib/kunit/.kunitconfig: warning: ignored by one of the .gitignore files
  mm/kfence/.kunitconfig: warning: ignored by one of the .gitignore files
  tools/testing/selftests/arm64/tags/.gitignore: warning: ignored by one of the .gitignore files
  tools/testing/selftests/arm64/tags/Makefile: warning: ignored by one of the .gitignore files
  tools/testing/selftests/arm64/tags/run_tags_test.sh: warning: ignored by one of the .gitignore files
  tools/testing/selftests/arm64/tags/tags_test.c: warning: ignored by one of the .gitignore files

These are ignored by the '.*' or 'tags' in the top .gitignore, but
there is no rule to negate it.

You might be tempted to do 'git add -f' but I want to have the real
issue fixed (by fixing a .gitignore, or by renaming files, etc.).

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>