Fix verifyCommits error when push a new branch (#26664) (#26810)

Backport #26664 by @CaiCandong

> ### Description
> If a new branch is pushed, and the repository has a rule that would
require signed commits for the new branch, the commit is rejected with a
500 error regardless of whether it's signed.
> 
> When pushing a new branch, the "old" commit is the empty ID
(0000000000000000000000000000000000000000). verifyCommits has no
provision for this and passes an invalid commit range to git rev-list.
Prior to 1.19 this wasn't an issue because only pre-existing individual
branches could be protected.
> 
> I was able to reproduce with
[try.gitea.io/CraigTest/test](https://try.gitea.io/CraigTest/test),
which is set up with a blanket rule to require commits on all branches.


Fix #25565
Very thanks to @Craig-Holmquist-NTI for reporting the bug and suggesting
an valid solution!

Co-authored-by: CaiCandong <50507092+CaiCandong@users.noreply.github.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

models/fixtures/email_address.yml

@@ -276,4 +276,12 @@
   email: user2-2@example.com
   lower_email: user2-2@example.com
   is_activated: false
-  is_primary: false
+  is_primary: false
+
+-
+  id: 36
+  uid: 36
+  email: abcde@gitea.com
+  lower_email: abcde@gitea.com
+  is_activated: true
+  is_primary: false