Extract method refactoring in class DirCacheCheckout
* added new public method getContent()
* changed checkoutEntry() to use the new getContent() method
* changed runExternalFilterCommand() parameter "entry" to "path" to be
independent from DirCacheEntry type
* removed runExternalFilterCommand() parameter "fs" as it can be derived
from repo
Bug: 356832
Change-Id: If8991b74870e0d8728093e98a990f93f1871b76a Signed-off-by: Andre Bossert <andre.bossert@siemens.com>