Tools: Fix the lint script

It turns out that last fix was only partial, and it didn't fix the
issue completely as paths were fixed, but staged changes were not
in the list. This should fix it for real, this time. Sorry about that…

(git diff needs --cached to include staged changes, unlike git status.)

Tools/lint.sh

@@ -5,7 +5,7 @@
 fail=0
 
 # Check for clang-format issues.
-for f in $(git diff --name-only --diff-filter=ACMRTUXB); do
+for f in $(git diff --name-only --diff-filter=ACMRTUXB --cached); do
   if ! echo "${f}" | egrep -q "[.](cpp|h|mm)$"; then
     continue
   fi