completion: factor out __git_resolve_builtins

completion: factor out __git_resolve_builtins

We're going to use the result of "git xxx --git-completion-helper" not
only for feeding COMPREPLY.

Therefore, factor out the execution and the caching of its results in
__gitcomp_builtin, to a new function __git_resolve_builtins.

While we're here, move an important comment we have in the function to
its header, so it gains visibility.

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>