changelog: move index_get_parents function up index_get_parents can be used in index_headrevs, to do so it needs to be moved up in the compilation unit.

mergecopies: avoid slowdown from linkrev adjustment (issue4680) checkcopies was using fctx.rev() which it was expecting would be equivalent to linkrev() but was triggering the new _adjustlinkrev path. This was making grafts and merges with large sets of potential copies very expensive.