Adds lazy option to git hook.

README.md

@@ -526,13 +526,18 @@ include the following in `.git/hooks/pre-commit`:
 import sys
 from isort.hooks import git_hook
 
-sys.exit(git_hook(strict=True, modify=True))
+sys.exit(git_hook(strict=True, modify=True, lazy=True))
 ```
 
 If you just want to display warnings, but allow the commit to happen
 anyway, call `git_hook` without the strict parameter. If you want to
 display warnings, but not also fix the code, call `git_hook` without the
 modify parameter.
+The `lazy` argument is to support users who are "lazy" to add files
+individually to the index and tend to use `git commit -a` instead.
+Set it to `True` to ensure all tracked files are properly isorted,
+leave it out or set it to `False` to check only files added to your
+index.
 
 Setuptools integration
 ----------------------

isort/hooks.py

@@ -33,7 +33,7 @@ def get_lines(command: List[str]) -> List[str]:
     return [line.strip() for line in stdout.splitlines()]
 
 
-def git_hook(strict: bool = False, modify: bool = False) -> int:
+def git_hook(strict: bool = False, modify: bool = False, lazy: bool = False) -> int:
     """
     Git pre-commit hook to check staged files for isort errors
 
@@ -43,12 +43,18 @@ def git_hook(strict: bool = False, modify: bool = False) -> int:
     :param bool modify - if True, fix the sources if they are not
         sorted properly. If False, only report result without
         modifying anything.
+    :param bool lazy - if True, also check/fix unstaged files.
+        This is useful if you frequently use ``git commit -a`` for example.
+        If False, ony check/fix the staged files for isort errors.
 
     :return number of errors if in strict mode, 0 otherwise.
     """
 
     # Get list of files modified and staged
     diff_cmd = ["git", "diff-index", "--cached", "--name-only", "--diff-filter=ACMRTUXB", "HEAD"]
+    if lazy:
+        diff_cmd.remove("--cached")
+
     files_modified = get_lines(diff_cmd)
     if not files_modified:
         return 0

tests/test_hooks.py

@@ -11,9 +11,25 @@ def test_git_hook(src_dir):
     # Ensure correct subprocess command is called
     with patch("subprocess.run", MagicMock()) as run_mock:
         hooks.git_hook()
-        assert run_mock.called_with(
-            ["git", "diff-index", "--cached", "--name-only", "--diff-filter=ACMRTUXB HEAD"]
-        )
+        assert run_mock.called_once()
+        assert run_mock.call_args[0][0] == [
+            "git",
+            "diff-index",
+            "--cached",
+            "--name-only",
+            "--diff-filter=ACMRTUXB",
+            "HEAD",
+        ]
+
+        hooks.git_hook(lazy=True)
+        assert run_mock.called_once()
+        assert run_mock.call_args[0][0] == [
+            "git",
+            "diff-index",
+            "--name-only",
+            "--diff-filter=ACMRTUXB",
+            "HEAD",
+        ]
 
     # Test with incorrectly sorted file returned from git
     with patch(