Partial fix for Issue #443: post-commit hook script (error) output lost
   This is step 3 : Introduce new type svn_commit_info_t that can hold
   all data required about a commit. Create a constructor too.

   * subversion/include/svn_types.h
     (svn_commit_info_t): New type.
     (svn_create_commit_info): New function. Constructor for
     svn_commit_info_t.

   * subversion/libsvn_subr/svn_helpers.c: New file.
     (svn_create_commit_info): New helper function. Constructor for
     svn_commit_info_t.