Prepend refs/heads/ to issue template refs (#20461) (#22427)

Backport #20461

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/context/repo.go

@@ -1087,6 +1087,9 @@ func (ctx *Context) IssueTemplatesErrorsFromDefaultBranch() ([]*api.IssueTemplat
 			if it, err := template.UnmarshalFromEntry(entry, dirName); err != nil {
 				invalidFiles[fullName] = err
 			} else {
+				if !strings.HasPrefix(it.Ref, "refs/") { // Assume that the ref intended is always a branch - for tags users should use refs/tags/<ref>
+					it.Ref = git.BranchPrefix + it.Ref
+				}
 				issueTemplates = append(issueTemplates, it)
 			}
 		}