Fix sometimes-spurious warning about changed config.h

After backing up and restoring config.h, `git diff-files` may report
it as potentially-changed because it isn't sure whether the index is
up to date. To avoid this, make sure that the git index is up-to-date.

This fixes the warning about changed config.h that you might get when
you run all.sh twice in succession, yet if you run `git status` or
`git diff` everything seems up to date and you no longer get the
warning because these git commands update the index.

https://stackoverflow.com/questions/36367190/git-diff-files-output-changes-after-git-status

tests/scripts/all.sh

@@ -387,7 +387,7 @@ pre_check_git () {
             exit 1
         fi
 
-        if ! git diff-files --quiet include/mbedtls/config.h; then
+        if ! git diff --quiet include/mbedtls/config.h; then
             err_msg "Warning - the configuration file 'include/mbedtls/config.h' has been edited. "
             echo "You can either delete or preserve your work, or force the test by rerunning the"
             echo "script as: $0 --force"