Teach git-notes about "notes.mergeStrategy" to select a general strategy
for all notes merges. This enables a user to always get expected merge
strategy such as "cat_sort_uniq" without having to pass the "-s" option
manually.
Signed-off-by: Jacob Keller <jacob.keller@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>