MINOR: pattern: implement pat_ref_load() to load a pattern at a given generation

MINOR: pattern: implement pat_ref_load() to load a pattern at a given generation

pat_ref_load() basically combines pat_ref_append() and pat_ref_commit().
It's very similar to pat_ref_add() except that it also allows to set the
generation ID and the line number. pat_ref_add() was modified to directly
rely on it to avoid code duplication. Note that a previous declaration
of pat_ref_load() was removed as it was just a leftover of an earlier
incarnation of something possibly similar, so no existing functionality
was changed here.