scripts/setlocalversion: git: Make -dirty check more robust

$(git diff-index) relies on the index being refreshed. This refreshing
of the index used to happen, but was removed in cdf2bc632ebc
("scripts/setlocalversion on write-protected source tree", 2013-06-14)
due to issues with a read-only filesystem.

If the index is not refreshed, one runs into problems. E.g. as
described in [0], git stores the uid in its index, so even if just the
uid has changed (or git is tricked into thinking so), then we will
think the tree is dirty. So as in [1], if you package linux-git with a
system that uses fakeroot(1), you get a "-dirty" version. Unless you
manually $(git update-index --refresh) themselves.

The simplest solution seems to be $(git status --porcelain), with an
additional flag saying "ignore untracked files". It seems clearer
about what it does, and avoids issues regarding cached indexes and
writable filesystems, but still has stable output for scripting.

[0]: https://public-inbox.org/git/0190ae30-b6c8-2a8b-b1fb-fd9d84e6dfdf@oracle.com/
[1]: https://bbs.archlinux.org/viewtopic.php?id=236702

Signed-off-by: Genki Sky <sky@genki.is>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

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