Filter unmodified files by default

Add `--diff-filter=M` to the git command that fetches changed files. The filter will ignore files that have been renamed or moved, but not had their content changed

src/darker/git.py

@@ -411,6 +411,7 @@ def _git_diff_name_only(
         "diff",
         "--name-only",
         "--relative",
+        "--diff-filter=M",
         rev1,
         # rev2 is inserted here if not WORKTREE
         "--",

src/darker/tests/conftest.py

@@ -73,6 +73,12 @@ def add(
             self._run("commit", "-m", commit)
         return absolute_paths
 
+    def rename(self, original_path: str, new_path: str, commit: str = None):
+        """Move/rename a file"""
+        self._run("mv", original_path, new_path)
+        if commit:
+            self._run("commit", "-m", commit)
+
     def get_hash(self, revision: str = "HEAD") -> str:
         """Return the commit hash at the given revision in the Git repository"""
         return self._run_and_get_first_line("rev-parse", revision)

src/darker/tests/test_git.py

@@ -608,16 +608,17 @@ def test_get_missing_at_revision_worktree(git_repo):
 
 
 def test_git_diff_name_only(git_repo):
-    """``_git_diff_name_only()`` only returns paths of modified files"""
+    """``_git_diff_name_only()`` only returns paths of modified files and ignores
+    renamed/moved files"""
     git_repo.add({"a.py": "a", "b.py": "b", "c.py": "c"}, commit="Initial commit")
     first = git_repo.get_hash()
     git_repo.add({"a.py": "A", "b.dy": "B"}, commit="only a.py modified")
+    git_repo.rename("c.py", "x.py", commit="rename c.py to x.py")
     second = git_repo.get_hash()
 
     result = git._git_diff_name_only(
         first, second, {Path("a.py"), Path("c.py"), Path("Z.py")}, git_repo.root
     )
-
     assert result == {Path("a.py")}
 
 

src/darker/tests/test_main.py

@@ -8,7 +8,6 @@
 import random
 import re
 import string
-import sys
 from argparse import ArgumentError
 from io import BytesIO
 from pathlib import Path