completion: let git provide the completable command list

completion: let git provide the completable command list

Instead of maintaining a separate list of command classification,
which often could go out of date, let's centralize the information
back in git.

While the function in git-completion.bash implies "list porcelain
commands", that's not exactly what it does. It gets all commands (aka
--list-cmds=main,others) then exclude certain non-porcelain ones. We
could almost recreate this list two lists list-mainporcelain and
others. The non-porcelain-but-included-anyway is added by the third
category list-complete.

Note that the current completion script incorrectly classifies
filter-branch as porcelain and t9902 tests this behavior. We keep it
this way in t9902 because this test does not really care which
particular command is porcelain or plumbing, they're just names.

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