kbuild: fix scripts/setlocalversion to avoid erroneous -dirty tag

If git's index file is out of date, and some files have been touched
such that their timestamp doesn't what is in the index, "git
diff-index HEAD" may show that a particular file is dirty, when in
fact it really isn't.  Running "git update-index" will update the
index to avoid these false positives.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>