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>