parse-options: convert bitfield values to use binary shift

parse-options: convert bitfield values to use binary shift

Because it's easier to read, but also likely to be easier to maintain.
I am making this change because I need to add a new flag in a later
commit.

Also add a trailing comma to the last enum entry to simplify addition of
new flags.

This change was originally suggested by Peff in:
https://public-inbox.org/git/YEZ%2FBWWbpfVwl6nO@coredump.intra.peff.net/

Signed-off-by: Andrzej Hunt <ajrhunt@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>