cmd/go/internal/modconv: fix TestConvertLegacyConfig expectations

It was a bug to find that commit in the Masterminds/semver repo.
It's not part of the main repo but only part of an unmerged pull request.
The code was updated to try not to look at unmerged pull requests,
but the test was not. Worse, whether the code succeeds at not looking
at unmerged pull requests apparently depends on the git version.
Sigh.

Fixes #26754.
Fixes #27043.

Change-Id: Ib9e07f565906de4f1169244911a258396688f14d
Reviewed-on: https://go-review.googlesource.com/129800


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>