[merge tracking] Merge conflict resolution callback API

For smoother merging on the merge-tracking branch, I'll be adding a
conflict resolution callback to the client API. This callback will be
used by svn_client_merge3() when a merge into an already-conflicted
file is encountered (eventually, I'll try to use it to whenever a
conflict occurs so that it can be resolved on-the-spot). Right now,
the API looks like this:

  typedef svn_error_t *(*svn_client_conflict_resolver_func_t)
    (const char *path,
     void *baton,
     apr_pool_t *pool);

Alternately, this API could return some sort of status indicating the
state of PATH after its resolution attempt, which could help keep the
code to actually perform the 'resolve' operation for a conflicted path
inside Subversion's core (instead of duplicated across client
implementations).

Now, should this API be a member of svn_client_ctx_t? If so, I
believe that I have to rev that data structure.

Otherwise, I can make it a parameter to svn_client_merge3() and
svn_client_merge_peg3().

Thoughts?