Add two additional custom keyword format codes which are used by FreeBSD.

   %H   - Equivalent to %P%_%r%_%d%_%a.
   %I   - Equivalent to %b%_%r%_%d%_%a.

With this, keyword expansion works as expected with svn.freebsd.org using
a stock Subversion client, rather than requiring a patched client.

These two codes are already in use at svn.freebsd.org and were part of the
originally submitted patch. I'd rather add support for these format codes
to Subversion than require FreeBSD to change its svn:keywords properties
throughout their repository history.

But I'm not sure whether adding additional alias format codes is a good
idea in general, so please don't regard this as a carte blanche for
everyone else to get their favorite format code aliases into the
Subversion code base.

* subversion/include/svn_subst.h
  (svn_subst_build_keywords3): Document support for %H and %I format codes.

* subversion/libsvn_subr/subst.c
  (keyword_printf): Implement support for %H and %I format codes.

* subversion/svn/svn.c
  (svn_cl__cmd_table): Document support for %H and %I in 'svn help propset'.

* subversion/tests/libsvn_subr/subst_translate-test.c
  (test_svn_subst_build_keywords3): Expand test to cover %H and %I.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1466598 13f79535-47bb-0310-9956-ffa450edef68