cmd/go/internal/modfetch/codehost: ignore incomplete semver tags in RecentTag

Fixes #31965

Change-Id: I2126903196b630c0bee2c022be1a818e0856ce3b
Reviewed-on: https://go-review.googlesource.com/c/go/+/176539
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/modfetch/codehost/git.go

@@ -678,7 +678,8 @@ func (r *gitRepo) RecentTag(rev, prefix string) (tag string, err error) {
 			}
 
 			semtag := line[len(prefix):]
-			if semver.IsValid(semtag) {
+			// Consider only tags that are valid and complete (not just major.minor prefixes).
+			if c := semver.Canonical(semtag); c != "" && strings.HasPrefix(semtag, c) {
 				highest = semver.Max(highest, semtag)
 			}
 		}