open-pr: allow update-scripts to stage changes already

The current logic calls for the `update-scripts` to leave uncommitted
changes. To be precise, it tests for _unstaged_ changes.

If an `update-scripts/versions/*` script already staged changes and did
not leave any unstaged changes around, the check would say:
"<package> already at <version>".

However, any new files need to be staged. Technically, we could stage
them via `git add -N`, but that would be only for the sake of
accommodating the current definition of the `open-pr` GitHub workflow,
when that workflow is just as easily improved instead.

So let's just enhance the check to look not only for unstaged changes,
but also for staged-yet-uncommitted changes.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

.github/workflows/open-pr.yml

@@ -109,7 +109,7 @@ jobs:
               PKGBUILD
           fi &&
           git update-index -q --refresh &&
-          if git diff-files --exit-code
+          if git diff-files --exit-code && git diff-index --quiet HEAD --
           then
             echo "::notice::$PACKAGE_TO_UPGRADE already at $UPGRADE_TO_VERSION"
             exit 0