Provide option for blame to use merged history

[Ryan J Ollos]

svn blame has the -g option:

  -g [--use-merge-history] : use/display additional information from merge
                             history

This can be useful for finding the changeset in which a line was changed, which is often more useful than the line in which the change was merged.

For example, line 47 of browser/trunk/trac/core.py shows [13672], however that's a merge changeset and [13671] may be more interesting.

svn blame -g trac/core.py on the trunk yields:

G  13671    rjollos         super(TracError, self).__init__(message)

while svn blame trac/core.py yields:

 13672    rjollos         super(TracError, self).__init__(message)

It looks like we could use blame4 or blame5 of svn.client, rather than blame2​:

    svn_client_blame4(*args)
        svn_client_blame4(char const * path_or_url, svn_opt_revision_t peg_revision, svn_opt_revision_t start, 
            svn_opt_revision_t end, svn_diff_file_options_t const * diff_options, svn_boolean_t ignore_mime_type, 
            svn_boolean_t include_merged_revisions, svn_client_blame_receiver2_t receiver, 
            void * receiver_baton, svn_client_ctx_t ctx, 
            apr_pool_t pool) -> svn_error_t
    
    svn_client_blame5(*args)
        svn_client_blame5(char const * path_or_url, svn_opt_revision_t peg_revision, svn_opt_revision_t start, 
            svn_opt_revision_t end, svn_diff_file_options_t const * diff_options, svn_boolean_t ignore_mime_type, 
            svn_boolean_t include_merged_revisions, svn_client_blame_receiver3_t receiver, 
            void * receiver_baton, svn_client_ctx_t ctx, 
            apr_pool_t pool) -> svn_error_t

Btw, I wonder if it would be useful to have a blame TracLink, or extend the browser TracLink to allow blame to be specified.