parse-options-cb: implement parse_opt_passthru_argv()

parse-options-cb: implement parse_opt_passthru_argv()

Certain git commands, such as git-pull, are simply wrappers around other
git commands like git-fetch, git-merge and git-rebase. As such, these
wrapper commands will typically need to "pass through" command-line
options of the commands they wrap.

Implement the parse_opt_passthru_argv() parse-options callback, which
will reconstruct all the provided command-line options into an
argv_array, such that it can be passed to another git command. This is
useful for passing command-line options that can be specified multiple
times.

Helped-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>