This 'tree-api' branch is a feature branch to develop a new API.
As a 'feature' branch, it is maintained by periodic 'catch-up' (aka
'sync') merges from trunk, and may one day be reintegrated to trunk.

See:
  https://cwiki.apache.org/confluence/display/SVN/Common+Tree+Interfaces

The new API is an abstraction of a generic tree of (usually) versioned
nodes -- where each node is a file or directory, with properties.  We
already have a generic tree-editing API (svn_delta_editor_t,
svn_editor_t).  It will be beneficial to have a generic tree-access API
as well.

The idea is that code can use the same interface to examine the WC base,
the WC working state, revision X in a repository, or any other concrete
or virtual tree that we may decide to implement.  Use of a common API
can reduce code duplication, improving consistency of behaviour,
simplifying maintenance and reducing bugs.

It should be possible to write operations that combine generic trees,
such as traversing two trees in parallel, diffing and patching trees.
Then we could, for example, extend 'svn diff' to be able to compare any
two arbitrary trees, related or not, such as the unversioned tree
'/tmp/saved-tests' against the WC tree 'wc/subversion/tests'.  Instead
of doing this by writing several more variants of the current three
subroutines (WC/WC, WC/repos, repos/repos), in principle we would need
just one diff subroutine that takes two Tree objects as input.