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If git's index file is out of date, and some files have been touched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>

scripts/setlocalversion

@@ -16,6 +16,7 @@ if head=`git rev-parse --verify HEAD 2>/dev/null`; then
 	fi
 
 	# Are there uncommitted changes?
+	git update-index --refresh --unmerged > /dev/null
 	if git diff-index HEAD | read dummy; then
 		printf '%s' -dirty
 	fi