Change "log -g" to use the ordinary svn_*_get_mergeinfo API (with

Change "log -g" to use the ordinary svn_*_get_mergeinfo API (with
include_descendents=TRUE) instead of its own special
svn_*_get_mergeinfo_for_tree API.  Also, improve some pool usage and
rename some variables.

This code is only tested by tests that are currently XFail, so I hope
it works!  That code doesn't crash, at least.

* subversion/libsvn_repos/log.c
  (get_combined_mergeinfo): Use svn_fs_get_mergeinfo with
   include_descendents=TRUE instead of svn_fs_get_mergeinfo_for_tree,
   and do it in a subpool.  Parse the returned strings.

git-svn-id: http://svn.collab.net/repos/svn/trunk@29009 612f8ebc-c883-4be0-9ee0-a4e9ef946e3a