cmd/go/internal/modload: fix sort condition in (*replacementRepo).Versions
In CL 258220 I added replacement versions to the repo versions used in
the modload.Query functions. The versions are computed from a map in
the modfile index, which has a nondeterministic iteration order.
I added a short-circuit condition to skip sorting in the (vastly
common) case where no replacement versions are added. However, while
cleaning up the change I accidentally deleted the line of code that
sets that condition. As a result, the test of that functionality
(mod_get_replaced) has been failing nondeterministically.
This change fixes the condition by comparing the slices before and
after adding versions, rather than by setting a separate variable.
The test now passes reliably (tested with -count=200).