Refactor context RefName and RepoAssignment (#33226)

The `ctx.Repo.RefName` was used to be a "short name", it causes a lot of
ambiguity.

This PR does some refactoring and use `RefFullName` to replace the
legacy `RefName`, and simplify RepoAssignment

modules/git/ref.go

@@ -80,6 +80,10 @@ func RefNameFromTag(shortName string) RefName {
 	return RefName(TagPrefix + shortName)
 }
 
+func RefNameFromCommit(shortName string) RefName {
+	return RefName(shortName)
+}
+
 func (ref RefName) String() string {
 	return string(ref)
 }