builtin/push: fix leaking refspec query result

builtin/push: fix leaking refspec query result

When appending a refspec via `refspec_append_mapped()` we leak the
result of `query_refspecs()`. The overall logic around refspec queries
is quite weird, as callers are expected to either set the `src` or `dst`
pointers, and then the (allocated) result will be in the respective
other struct member.

As we have the `src` member set, plugging the memory leak is thus as
easy as just freeing the `dst` member. While at it, use designated
initializers to initialize the structure.

This leak was exposed by t5516, but fixing it is not sufficient to make
the whole test suite leak free.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>