Fix missing no-change message in -ie mode

This warning is nice to see,
but only worked in `-i` and `-e` mode, not in `-ie` mode:

    (warning) no changes performed

The Step.message was initialized to None instead of the old value,
causing unedited messages in message-edit mode to compare unequal.

This commit removes the optionality and thereby the possibility
for the problem.
It can also be solved by interpreting None as the old value
or resetting the new value to None if it is indifferent.

gitrevise/todo.py

@@ -41,12 +41,12 @@ def parse(instr: str) -> StepKind:
 class Step:
     kind: StepKind
     commit: Commit
-    message: Optional[bytes]
+    message: bytes
 
     def __init__(self, kind: StepKind, commit: Commit) -> None:
         self.kind = kind
         self.commit = commit
-        self.message = None
+        self.message = commit.message
 
     @staticmethod
     def parse(repo: Repository, instr: str) -> Step:

tests/test_interactive.py

@@ -1,4 +1,4 @@
-from typing import TYPE_CHECKING, Optional, Sequence
+from typing import TYPE_CHECKING, List, Optional, Sequence
 
 import pytest
 
@@ -343,3 +343,22 @@ def test_interactive_reword(repo: Repository) -> None:
     assert prev_u.tree().entries[b"file2"] == curr.tree().entries[b"file2"]
     assert prev_u.tree().entries[b"file1"] == curr_uu.tree().entries[b"file1"]
     assert prev.tree().entries[b"file1"] == curr_u.tree().entries[b"file1"]
+
+
+@pytest.mark.parametrize("interactive_mode", ["-i", "-ie", "-e"])
+def test_no_changes(repo: Repository, interactive_mode: str) -> None:
+    bash("git commit --allow-empty -m empty")
+    old = repo.get_commit("HEAD")
+    assert old.message == b"empty\n"
+
+    base = "--root" if interactive_mode != "-e" else "HEAD"
+
+    outputs: List[bytes] = []
+    with editor_main([interactive_mode, base], stdout_stderr_out=outputs) as ed:
+        with ed.next_file() as f:
+            f.replace_dedent(f.indata)
+
+    normalized_outputs = [text.decode().replace("\r\n", "\n") for text in outputs]
+    assert normalized_outputs == ["", "(warning) no changes performed\n"]
+    new = repo.get_commit("HEAD")
+    assert new.oid == old.oid