diff.c: prepare to use parse_options() for parsing

diff.c: prepare to use parse_options() for parsing

This is a preparation step to start using parse_options() to parse
diff/revision options instead of what we have now. There are a couple
of good things from using parse_options():

- better help usage
- easier to add new options
- better completion support
- help usage generation
- better integration with main command option parser. We can just
  concat the main command's option array and diffopt's together and
  parse all in one go.
- detect colidding options (e.g. --reverse is used by revision code,
  so diff code can't use it as long name for -R)
- consistent syntax, e.g. option that takes mandatory argument will
  now accept both "--option=value" and "--option value".

The plan is migrate all diff/rev options to parse_options(). Then we
could get rid of diff_opt_parse() and expose parseopts[] directly to
the caller.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>