Add support for custom keyword definitions, fixing issue #890.

Custom keywords can be defined using a format string which controls
the way keyword information is expanded. For instance, a keyword set
the command 'svn propset svn:keywords MyKeyword=%a%_%b%_%d myfile'
can be referenced in 'myfile' as $MyKeyword$, and expands to the author,
a space, the file's name ('myfile'), another space, and the short
representation of the date of the last-changed revision.

The following format codes are currently available:

  %a   - The author.
  %b   - The basename of the URL.
  %d   - Short format of the date.
  %D   - Long format of the date.
  %P   - The file's path, relative to the repository root URL.
  %r   - The revision.
  %R   - The URL to the root of the repository.
  %u   - The URL of the file.
  %_   - A space (keyword definitions cannot contain a literal space).
  %    - A literal '%'.

Most of these were already defined internally in libsvn_subr.
This patch adds the %P, %R, and %_ format codes.
More format codes could be added in the future but might require API changes.

Based on a patch submitted several times by various FreeBSD developers
who have added this feature to their port of Subversion.
I'm crediting those who I know were involved in this patch in one way
or another. But I don't know for sure who wrote the initial implementation.

Patch by: Peter Wemm <peter@FreeBSD.org>
          David O'Brien <obrien@FreeBSD.org>
          Lev A. Serebryakov <lev@FreeBSD.org>
          Alfred Perlstein <alfred@FreeBSD.org>
          me

* subversion/include/svn_subst.h
  (svn_subst_build_keywords3): Declare.
  (svn_subst_build_keywords2): Deprecate.

* subversion/libsvn_client/cat.c
  (svn_client__get_normalized_stream): Call svn_subst_build_keywords3().

* subversion/libsvn_client/export.c
  (export_node, close_file): Call svn_subst_build_keywords3().

* subversion/libsvn_client/import.c
  (send_file_contents): Call svn_subst_build_keywords3().

* subversion/libsvn_subr/subst.c
  (keyword_printf): Add support for %P, %R, and %_. Add repos_root_url
   parameter which is required to implement these new format codes.
  (build_keywords): New helper function which implements the guts of
   svn_subst_build_keywords2() and svn_subst_build_keywords3().
  (svn_subst_build_keywords2): Reimplement as wrapper around build_keywords().
    Turn off support for custom keywords to ensure backwards compatibility.
  (svn_subst_build_keywords3): New, implemented as wrapper around the
   build_keywords() helper. Enable support for custom keywords.

* subversion/libsvn_wc/translate.c
  (svn_wc__expand_keywords): Call svn_subst_build_keywords3().

* subversion/svn/svn.c
  (svn_cl__cmd_table): Document the custom keywords feature, including the
   support format codes, in the output of 'svn help propset'.

* subversion/tests/libsvn_subr/subst_translate-test.c
  (test_svn_subst_build_keywords3): New test.
  (test_funcs): Add new test.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1466570 13f79535-47bb-0310-9956-ffa450edef68