notes: teach git-notes about notes.<name>.mergeStrategy option

notes: teach git-notes about notes.<name>.mergeStrategy option

Teach notes about a new "notes.<name>.mergeStrategy" option for
configuring the notes merge strategy when merging into
refs/notes/<name>. This option allows for the selection of merge
strategy for particular notes refs, rather than all notes ref merges, as
user may not want cat_sort_uniq for all refs, but only some. Note that
the <name> is the local reference we are merging into, not the remote
ref we merged from. The assumption is that users will mostly want to
configure separate local ref merge strategies rather than strategies
depending on which remote ref they merge from.

notes.<name>.mergeStrategy overrides the general behavior as it is more
specific.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>