Do not leave dead transaction if commit is blocked by start-commit hook. Also
fix svn_repos_fs_begin_txn_for_commit2() API promise regression that was 
broken in r1376201.

* subversion/libsvn_repos/fs-wrap.c
  (svn_repos_fs_begin_txn_for_commit2): Abort created transaction if error 
   happens between transaction creation and successful return from 
   function as promised in docstring. Also keep output *TXN_P 
   parameter unaffected on failure as promised in docstring.

* subversion/tests/cmdline/commit_tests.py
  (start_commit_hook_test): Verify that there is no dead transaction left 
   when commit was blocked by start-commit hook.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1583977 13f79535-47bb-0310-9956-ffa450edef68