Fix the svn_client_get_merging_summary() function, which would crash or throw
an assertion failure if called with the target being a non-absolute WC path
and the revision kind being 'working' or 'unspecified'.  The current 'svn'
never calls it this way, but other clients could (and I tried writing some
code that did).

* subversion/libsvn_client/merge.c
  (find_automatic_merge): Add a proper doc string.
  (find_automatic_merge_no_wc): Document the output.
  (client_find_automatic_merge): Assert that the 'target_abspath' input is
    indeed an abspath. Initialize the 'merge->target' output.
  (svn_client_get_merging_summary): Convert the path-or-URL to an abspath
    when required.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1532396 13f79535-47bb-0310-9956-ffa450edef68