Partial fix for Issue #443: post-commit hook script (error) output lost
   Includes post-commit hook stderr propagation for ra_dav
   This is step 1 : Introduction of svn_client_commit_info2_t and its
   constructor.

   * subversion/include/svn_client.h
     (svn_client_commit_info2_t): New structure to take care of
     post-commit-hook's stderr.
     (svn_client_create_commit_info): New fuction. Constructor for
     svn_client_commit_info2_t.

   * subversion/libsvn_client/commit.c
     (svn_client_create_commit_info): New function.