send-pack: fix memory leak around duplicate refs

The 'git-send-pack(1)' allows users to push objects to a remote repository and explicitly list the references to be pushed. The status of each reference pushed is captured into a list mapped by refname. If a reference fails to be updated, its error message is captured in the `ref->remote_status` field. While the command allows duplicate ref inputs, the list doesn't accommodate this behavior as a particular refname is linked to a single `struct ref*` element. So if the user inputs a reference twice like: git send-pack remote.git A:foo B:foo where the user is trying to update the same reference 'foo' twice and the reference fails to be updated, we first fill `ref->remote_status` with error message for the input 'A:foo' then we override the same field with the error message for 'B:foo'. This override happens without first free'ing the previous value. Fix this leak. The current tests already incorporate the above example, but in the test 'A:foo' succeeds while 'B:foo' fails, meaning that the memory leak isn't triggered. Add a new test with multiple duplicates. Signed-off-by: Karthik Nayak <karthik.188@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>