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

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: wxiaoguang <wxiaoguang@gmail.com>

modules/git/repo_compare.go

@@ -280,8 +280,16 @@ func (repo *Repository) GetPatch(base, head string, w io.Writer) error {
 }
 
 // GetFilesChangedBetween returns a list of all files that have been changed between the given commits
+// If base is undefined empty SHA (zeros), it only returns the files changed in the head commit
+// If base is the SHA of an empty tree (EmptyTreeSHA), it returns the files changes from the initial commit to the head commit
 func (repo *Repository) GetFilesChangedBetween(base, head string) ([]string, error) {
-	stdout, _, err := NewCommand(repo.Ctx, "diff", "--name-only", "-z").AddDynamicArguments(base + ".." + head).RunStdString(&RunOpts{Dir: repo.Path})
+	cmd := NewCommand(repo.Ctx, "diff-tree", "--name-only", "--root", "--no-commit-id", "-r", "-z")
+	if base == EmptySHA {
+		cmd.AddDynamicArguments(head)
+	} else {
+		cmd.AddDynamicArguments(base, head)
+	}
+	stdout, _, err := cmd.RunStdString(&RunOpts{Dir: repo.Path})
 	if err != nil {
 		return nil, err
 	}