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/HEAD

@@ -0,0 +1 @@
+ref: refs/heads/main

routers/private/tests/repos/repo1_hook_verification/config

@@ -0,0 +1,6 @@
+[core]
+	repositoryformatversion = 0
+	filemode = false
+	bare = true
+	symlinks = false
+	ignorecase = true

routers/private/tests/repos/repo1_hook_verification/info/refs

@@ -0,0 +1 @@
+d766f2917716d45be24bfa968b8409544941be32	refs/heads/main

routers/private/tests/repos/repo1_hook_verification/logs/HEAD

@@ -0,0 +1 @@
+0000000000000000000000000000000000000000 d766f2917716d45be24bfa968b8409544941be32 Gitea <gitea@fake.local> 1693148474 +0800	push

routers/private/tests/repos/repo1_hook_verification/logs/refs/heads/main

@@ -0,0 +1 @@
+0000000000000000000000000000000000000000 d766f2917716d45be24bfa968b8409544941be32 Gitea <gitea@fake.local> 1693148474 +0800	push

routers/private/tests/repos/repo1_hook_verification/objects/72/920278f2f999e3005801e5d5b8ab8139d3641c

@@ -0,0 +1,2 @@
+x
•ŽK
+1]çÙÒéüAÄS¸ï$Í"32ooð®ŠWð òÞ{›!žæ`–˜JC%¡.˜$Ár]sѱe$ïmòâMƒ·)£÷±(O`ªbtlÐE[:;4–àHÐ1_û<5F>”rayýáþl“é’÷~“ÊE­L@cå€Xv…Mþã":µMÛƒG«_}À?Ý

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

@@ -0,0 +1,2 @@
+x
•<>1
+!ES{ŠéAwGGa	9EúQgW·Èí#¹AªÞû©ÕZ§/£‹€³Œ–p±ì(¤(<28>ó®óBhÈÛ¼&ᙟãÝ:pLY`ûÍãU†ð-µzŸÁ°ô†\µ×ZM:<3A>†ü¡¨Êå€óxJ/ûG}
:µ3

routers/private/tests/repos/repo1_hook_verification/objects/cb/a4c30c196a0e03e7bdf6eeb8393d14b9d073aa

@@ -0,0 +1,3 @@
+x
•ŽA
+Â0E]ç³$™L“ˆx•L2µ]´•<C2B4>
+ÞÞê

routers/private/tests/repos/repo1_hook_verification/objects/d7/66f2917716d45be24bfa968b8409544941be32

@@ -0,0 +1,3 @@
+x
•’Ë®«FE3æ+zn%44æ!%Qxƒ<78>Û€
s˜AÓ`8Øæëã{£Ì2IM¶j•ª´¥Údèûf²Ìý2<C3BD>”‚"‡$§e‰¶
+-(â ­Ä!´ÝJ"åaŲ@•BaîùHo3 ŸVØòå<$<24>/)å$JøJD’B¡•H¤§˜ü{¾#ÈRRðûOù«nfšÿF†þOÀ‰
+âq[°<>2„̇~ŒÍô¬Ô÷zjjðë<C3B0>ÒLÛÅÀ·}prm¬Fqhþä`@Ø«¦ªš®ª¥Õ˜fî?3Ç[7Š³…ê¨Ð)	^™þuÿÖ¿,µ<>Æl7©zÝÿr|&«Ou4<75>Ø9Ó:µÎQjôû·êÕ1x±õå6ÍQ‡÷ƒÀ%Áåtû‰sò¸íV‰|( V¿<56>,aL,ù«G~²Ç<16>¹‹<C2B9>‹r¥ùûî@·`·Àþ$[!	XËŠep©Œæ[8	oýä(›« k£Z´Î³yóeйÙÆÄ«Y²¿kÖd€¯6•3¾;3ÜÔ	RÔiÞ‹dYÓDk91V]/Cê#º¾&ÿêpo´Fáb¯‹¶}§¹ô¦òuW&]+m xaqdÜIõX¯þ3

routers/private/tests/repos/repo1_hook_verification/objects/info/packs

@@ -0,0 +1 @@
+

routers/private/tests/repos/repo1_hook_verification/objects/tmp_objdir-incoming-a21648/cb/a4c30c196a0e03e7bdf6eeb8393d14b9d073aa

@@ -0,0 +1,3 @@
+x
•ŽA
+Â0E]ç³$™L“ˆx•L2µ]´•<C2B4>
+ÞÞê

routers/private/tests/repos/repo1_hook_verification/refs/heads/main

@@ -0,0 +1 @@
+d766f2917716d45be24bfa968b8409544941be32