argv-array.h

When we have a null-terminated array, it would be useful to convert it
or append it to an argv_array for further manipulation.

Implement argv_array_pushv() which will push a null-terminated array of
strings on to an argv_array.

Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>