Replace conflicted file content accessor macros with a single function.
The accessors svn_client_conflict_get_base_abspath(), svn_client_conflict_get_my_abspath(), and svn_client_conflict_get_their_abspath() are replaced by a new function called svn_client_conflict_text_get_contents(). For now, this function returns the same outputs (absolute paths) but this may change in future.
* subversion/libsvn_client/resolved.c (svn_client_conflict_text_get_contents): Implement. This implementation is not complete yet since the WC base content it could return is not always available in the svn_wc_conflict_description2_t struct.