fix(pull): name the head repository in default compare links (#39075)

The "New Pull Request" buttons and the `/pulls/new/{branch}` redirect
build their compare link as `{owner}:{branch}`. If a fork and its parent
share an owner, through ALLOW_FORK_INTO_SAME_OWNER, or after a transfer,
that head resolves back to the base repo, so the link compares the base
against itself and 404s on a branch that only exists in the fork.

Switching to `{owner}/{repo}:{branch}` names the head repo
unambiguously, and it's what the compare page's own links already use.

Also clears the 404 in #37649; the archived-parent half of that report
is separate.

routers/web/repo/pull.go

@@ -1308,7 +1308,7 @@ func PullsNewRedirect(ctx *context.Context) {
 			return
 		}
 		redirectRepo = repo.BaseRepo
-		branch = fmt.Sprintf("%s:%s", repo.OwnerName, branch)
+		branch = context.CompareHeadRef(repo, branch)
 	}
 	ctx.Redirect(fmt.Sprintf("%s/compare/%s...%s?expand=1", redirectRepo.Link(), util.PathEscapeSegments(redirectRepo.DefaultBranch), util.PathEscapeSegments(branch)))
 }