git-completion.bash: introduce __gitcomp_builtin

git-completion.bash: introduce __gitcomp_builtin

This is a __gitcomp wrapper that will execute

    git ... --git-completion-helper

to get the list of completable options. The call will be made only
once and cached to avoid performance issues, especially on Windows.

__gitcomp_builtin() allows callers to change its output a bit by adding
some more options, or removing some.

- Current --git-completion-helper for example does not output --no-foo
  form, this has to be added manually by __gitcomp_builtin() callers
  when necessary

- Some options from --git-completion-helper should only be available in
  certain conditions (e.g. --continue and friends). __gitcomp_builtin()
  callers can remove them if the conditions are not met.

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