cmd/go/internal/modfetch/gitrepo: fix bug loading tags
cmd/go/internal/modfetch/gitrepo: fix bug loading tags

This was introduced by the r.remote -> origin cleanup
and breaks TestConvertLegacyConfig.

Change-Id: I877e9ecd39b86e240bc696a4c7c7e13b40e2079d
Reviewed-on: https://go-review.googlesource.com/117355
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

golang/vgo,golang/vgo