Re-use ra sessions when determining where a path is copied from using a
libsvn_client internal api. Give public api a caller from our test suite
by adding a simple C test.

* subversion/libsvn_client/client.h
  (svn_client__get_copy_source): Add argument.

* subversion/libsvn_client/log.c
  (svn_client__get_copy_source): Add support for using an existing session.

* subversion/libsvn_client/merge.c
  (normalize_merge_sources_internal): Pass existing ra session.

* subversion/libsvn_client/mergeinfo.c
  (svn_client_suggest_merge_sources): Open ra session and pass it to two
    functions instead of having them create and close their own sessions.
    Use subpool.

* subversion/tests/libsvn_client/client-test.c
  (test_suggest_mergesources): New test.
  (test_funcs): Add test_suggest_mergesources.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1552265 13f79535-47bb-0310-9956-ffa450edef68