ResolveMerger: Allow setting the TreeWalk AttributesNodeProvider

When a merger is created without a Repository, no
AttributesNodeProvider is created in the TreeWalk. Since mergers are
often created with a custom ObjectInserter and no repo, they skip any
lookups of attributes from any of the gitattributes files (within a
tree, in the repo info/ dir, or user/global). Since there are
potentially merge-affecting attributes in those files, callers might
want to use both a custom ObjectInserter and an AttributesNodeProvider.

Change-Id: I7997309003bbb598e1002261b3be7f2cc52066c8