argv-array: implement argv_array_pushv()

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>

Documentation/technical/api-argv-array.txt

@@ -46,6 +46,9 @@ Functions
 	Format a string and push it onto the end of the array. This is a
 	convenience wrapper combining `strbuf_addf` and `argv_array_push`.
 
+`argv_array_pushv`::
+	Push a null-terminated array of strings onto the end of the array.
+
 `argv_array_pop`::
 	Remove the final element from the array. If there are no
 	elements in the array, do nothing.