AddCommand: Use NameConflictTreeWalk to identify file-dir changes

AddCommand: Use NameConflictTreeWalk to identify file-dir changes

Adding a path that already exists but is changing type such as
from symlink to subdirectory requires a NameConflictTreeWalk to
match up the two different entry types that share the same name.

NameConflictTreeWalk needs a bug fix to pop conflicting entries
when PathFilterGroup aborts the walk early so that it does not
allow DirCacheBuilderIterator to copy conflicting entries into
the output cache.

Change-Id: I61b49cbe949ca8b4b98f9eb6dbe7b1f82eabb724