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>

routers/private/tests/repos/repo1_hook_verification/objects/97/79d17a04f1e2640583d35703c62460b2d86e0a

@@ -0,0 +1,2 @@
+x
<01><>1
+!ES{<7B><>AwGGa	9E<39>QgW<10><><EFBFBD>#<23>A<EFBFBD><0F><><EFBFBD><EFBFBD>Z<18>/<2F><><EFBFBD><EFBFBD><EFBFBD><EFBFBD>p<EFBFBD><70>(<28>(<28><><EFBFBD><EFBFBD>Bh<42>ۼ&<26><11><14><><EFBFBD>:pLY`<60><><EFBFBD>U<EFBFBD><55>-<2D>z<EFBFBD><7A><EFBFBD><1A><>\<5C><>ZM:<3A><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>xJ/<2F>G}
:<3A>3