optimize compatibility checks in merge.unique_variable

[dcherian]

Currently merge.unique_variable does this

    if equals is None:
        out = out.compute()
        for var in variables[1:]:
            equals = getattr(out, compat)(var)
            if not equals:
                break

out (=variables[0]) is always computed though it may not be necessary.

One solution would be to loop through once checking attrs, shapes and _data. If these checks were satisfied, then we execute the above code