RelNotes 2.18: clarify where directory rename detection applies

Mention that this feature works with some commands (merge and cherry-pick, implying that it also works with commands that build on these like rebase -m and rebase -i). Explicitly mentioning two commands hopefully implies that it may not always work with other commands (am, and rebase without flags that imply either -m or -i). Also, since the directory rename detection from this cycle was specifically added in merge-recursive and not diffcore-rename, remove the 'in "diff" family" phrase from the note. (Folks have requested in the past that `git diff` detect directory renames and somehow simplify its output, so it may be helpful to avoid implying that diff has any new capability here.) Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>