argv-array: return the pushed string from argv_push*()

argv-array: return the pushed string from argv_push*()

Such an API change allows us to use an argv_array this way:

    struct argv_array to_free = ARGV_ARRAY_INIT;
    const char *msg;

    if (some condition) {
            msg = "constant string message";
            ... other logic ...
    } else {
            msg = argv_array_pushf(&to_free, "format %s", var);
    }
    ... use "msg" ...
    ... do other things ...
    argv_array_clear(&to_free);

Note that argv_array_pushl() and argv_array_pushv() are used to push
one or more strings with a single call, so we do not return any one
of these strings from these two functions in order to reduce the
chance to misuse the API.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>