Support getting changed files when commit ID is EmptySHA (#26290) (#26316)

Backport #26290 by @Zettat123

Fixes #26270.

Co-Author: @wxiaoguang

Thanks @lunny for providing this solution

As
https://github.com/go-gitea/gitea/issues/26270#issuecomment-1661695151
said, at present we cannot get the names of changed files correctly when
the `OldCommitID` is `EmptySHA`. In this PR, the `GetCommitFilesChanged`
method is added and will be used to get the changed files by commit ID.

References:
- https://stackoverflow.com/a/424142

Co-authored-by: Zettat123 <zettat123@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
(cherry picked from commit a57568bad7)

modules/git/sha1.go

@@ -11,10 +11,10 @@ import (
 	"strings"
 )
 
-// EmptySHA defines empty git SHA
+// EmptySHA defines empty git SHA (undefined, non-existent)
 const EmptySHA = "0000000000000000000000000000000000000000"
 
-// EmptyTreeSHA is the SHA of an empty tree
+// EmptyTreeSHA is the SHA of an empty tree, the root of all git repositories
 const EmptyTreeSHA = "4b825dc642cb6eb9a060e54bf8d69288fbee4904"
 
 // SHAFullLength is the full length of a git SHA