builtin-notes: Refactor handling of -F option to allow combining -m and -F

builtin-notes: Refactor handling of -F option to allow combining -m and -F

By moving the -F option handling into a separate function (parse_file_arg),
we can start allowing several -F options, and mixed usage of -m and -F
options. Each -m/-F given appends to the note message, in the order they are
given on the command-line.

The patch includes tests verifying correct behaviour of the new functionality.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>