graftcopies: document why the function is useful at all Despite having spent a significant amount on time on the copy-tracing code, I thought `graftcopies()` (formerly known as `duplicatecopies()`) was needed to duplicate copies after calling `merge.update()` to do a merge (as `merge.graft()` does), but it's actually usually not needed; `merge.update()` takes care of most copies. This patch documents what the function is for. Differential Revision: https://phab.mercurial-scm.org/D7861