test/test-migrate-fixtures.sh: remove invalid '--create'
test/test-migrate-fixtures.sh: remove invalid '--create'

When invoking 'git update-ref', a user needs only give the 'create'
option when also invoked with --stdin.

Since we are not invoking with --stdin, the meaning of --create is
implicitly understood. Instead, let's avoid passing a non-existent
option and invoke the command correctly.

github/git-lfs,github/git-lfs,github/git-lfs