Match tar's "--exclude-caches"-semantic

Match tar's "--exclude-caches"-semantic

With --exclude-caches, only exclude the directory contents
(without the tag-file itself), thus matching tar's semantic

(The previous implementation matched tar's "--exclude-caches-all")

(cherr-picked from proposed/index-exclude-caches branch: e1f95c0d)
Signed-off-by: Tim Riemenschneider <git@tim-riemenschneider.de>