Refactor hash access in svn_config_t code.  Provide a utility function
that minimizes the overhead for case-sensitive access.

* subversion/libsvn_subr/config.c
  (get_hash_value): new utility function
  (find_option,
   svn_config_has_section): use it to simplify code & speed things up

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1538068 13f79535-47bb-0310-9956-ffa450edef68
On the invoke-diff-cmd-feature branch: trunk revision 1538068 merged
into branch revision 1538050.  Correct errors trying to revert and
re-applied all the changes from r1537987: Continuation from r1535411:
Adjust substitution syntax to %svn_(label)_[old|new|mine|yours|base].
Update string assigment to use apr_pstrdup.  Update tests. Update help
info.

* subversion/libsvn_subr/io.c

  (__create_custom_diff_cmd): Update substitution syntax.  Update
    string assigment to use apr_pstrdup.

  (svn_io_run_diff2): Update default user args syntax to reflect new
    substitution syntax.

* subversion/svn/svn.c
  (svn_cl__options[], "invoke-diff-cmd"): Update help info.
  
* BRANCH-README: Update.

* /subversion/tests/cmdline/diff_tests.py: Update tests.



git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/invoke-diff-cmd-feature@1538071 13f79535-47bb-0310-9956-ffa450edef68