git notes merge: Initial implementation handling trivial merges only

git notes merge: Initial implementation handling trivial merges only

This initial implementation of 'git notes merge' only handles the trivial
merge cases (i.e. where the merge is either a no-op, or a fast-forward).

The patch includes testcases for these trivial merge cases.

Future patches will extend the functionality of 'git notes merge'.

This patch has been improved by the following contributions:
- Stephen Boyd: Simplify argc logic
- Stephen Boyd: Use test_commit
- Ævar Arnfjörð Bjarmason: Don't use C99 comments.
- Jonathan Nieder: Add constants for common verbosity values
- Jonathan Nieder: Use trace_printf(...) instead of OUTPUT(o, 5, ...)
- Jonathan Nieder: Remove extraneous show() function
- Jonathan Nieder: Clarify handling of empty/missing notes ref in notes_merge()
- Junio C Hamano: fixup minor style issues

Thanks-to: Stephen Boyd <bebarino@gmail.com>
Thanks-to: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Thanks-to: Jonathan Nieder <jrnieder@gmail.com>
Thanks-to: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>