This fixes a regression of 5b85fba0: diff-index is porcelain and seems
to lack some explicit refresh prior to being called. Therefore, it can
incorrectly detect files as being modified although a "git diff" will
not - and will actually also update the status so that the next
diff-index would be fine again.
Fix those false positives while still detecting also cached changes by
using git status -s instead.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
76e7339fdb