GitChangeSet.getBranch always returned null, make it explicit

The branch instance variable was not initialized and was never
written.  The Java language spec declares that instance variables are
initialized with their default value (null for an object).  Thus,
getBranch() always returned null.

This change removes the branch instance variable which was never
assigned.

Found by findbugs
Remove unread field from SubmoduleCOmbinator
Exclude wrong map iterator findbugs warning on SubmoduleCombinator

Needed due to logic of the difference() method