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

> ### 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: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

models/fixtures/user.yml

@@ -1301,7 +1301,7 @@
   lower_name: limited_org36
   name: limited_org36
   full_name: Limited Org 36
-  email: limited_org36@example.com
+  email: abcde@gitea.com
   keep_email_private: false
   email_notifications_preference: enabled
   passwd: ZogKvWdyEx:password
@@ -1320,7 +1320,7 @@
   allow_create_organization: true
   prohibit_login: false
   avatar: avatar22
-  avatar_email: limited_org36@example.com
+  avatar_email: abcde@gitea.com
   use_custom_avatar: false
   num_followers: 0
   num_following: 0