pre-commit: uncrustify based on staged changes

pre-commit: uncrustify based on staged changes

Previously the generated patch was based on the file(s) in the working
directory. This is a problem if you have not to be commited changes
there and these changes fix formatting issues that exist in the staging
area. This effectively circumventes the script from rejecting the
commit.

An example:
   git add file.c
   git commit
   ... pre-commit hooks complains about formatting ...
   ... you fix the file manually, forget to git add ...
   git commit
   ... succeeds, even though the commit still has issues ...

Signed-off-by: Heiko Hund <heiko@ist.eigentlich.net>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Antonio Quartulli <a@unstable.cc>
Message-Id: <20220517210121.1312072-1-heiko@ist.eigentlich.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg24376.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>