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_test.go

@@ -119,3 +119,42 @@ func TestReadWritePullHead(t *testing.T) {
 	err = repo.RemoveReference(PullPrefix + "1/head")
 	assert.NoError(t, err)
 }
+
+func TestGetCommitFilesChanged(t *testing.T) {
+	bareRepo1Path := filepath.Join(testReposDir, "repo1_bare")
+	repo, err := openRepositoryWithDefaultContext(bareRepo1Path)
+	assert.NoError(t, err)
+	defer repo.Close()
+
+	testCases := []struct {
+		base, head string
+		files      []string
+	}{
+		{
+			EmptySHA,
+			"95bb4d39648ee7e325106df01a621c530863a653",
+			[]string{"file1.txt"},
+		},
+		{
+			EmptySHA,
+			"8d92fc957a4d7cfd98bc375f0b7bb189a0d6c9f2",
+			[]string{"file2.txt"},
+		},
+		{
+			"95bb4d39648ee7e325106df01a621c530863a653",
+			"8d92fc957a4d7cfd98bc375f0b7bb189a0d6c9f2",
+			[]string{"file2.txt"},
+		},
+		{
+			EmptyTreeSHA,
+			"8d92fc957a4d7cfd98bc375f0b7bb189a0d6c9f2",
+			[]string{"file1.txt", "file2.txt"},
+		},
+	}
+
+	for _, tc := range testCases {
+		changedFiles, err := repo.GetFilesChangedBetween(tc.base, tc.head)
+		assert.NoError(t, err)
+		assert.ElementsMatch(t, tc.files, changedFiles)
+	}
+}