split out DiffTreeItem

This makes github-backup happier when it reuses this library.
Work around statfs() overflow on some XFS systems.

statfs(".", 0xffa8ad50)                 = -1 EOVERFLOW (Value too large for defined data type)

Ref <20141222221621.GO7251@onerussian.com>
Merge branch 'master' of ssh://git-annex.branchable.com