Avoid warnings when not running inside git repos

Avoid confusing user with warning messages when linter is run outside
git repos. This change will silently fallback to os.walk when run
outside git repos.

Fixes: #1410

src/ansiblelint/file_utils.py

@@ -212,10 +212,11 @@ def get_yaml_files(options: Namespace) -> Dict[str, Any]:
             git_command, stderr=subprocess.STDOUT, universal_newlines=True
         ).split("\x00")[:-1]
     except subprocess.CalledProcessError as exc:
-        _logger.warning(
-            "Failed to discover yaml files to lint using git: %s",
-            exc.output.rstrip('\n'),
-        )
+        if not (exc.returncode == 128 and 'fatal: not a git repository' in exc.output):
+            _logger.warning(
+                "Failed to discover yaml files to lint using git: %s",
+                exc.output.rstrip('\n'),
+            )
     except FileNotFoundError as exc:
         if options.verbosity:
             _logger.warning("Failed to locate command: %s", exc)
@@ -253,10 +254,19 @@ def guess_project_dir() -> str:
 
 def expand_dirs_in_lintables(lintables: Set[Lintable]) -> None:
     """Return all recognized lintables within given directory."""
-    all_files = get_yaml_files(options)
+    should_expand = False
 
-    for item in copy.copy(lintables):
+    for item in lintables:
         if item.path.is_dir():
-            for filename in all_files:
-                if filename.startswith(str(item.path)):
-                    lintables.add(Lintable(filename))
+            should_expand = True
+            break
+
+    if should_expand:
+        # this relies on git and we do not want to call unless needed
+        all_files = get_yaml_files(options)
+
+        for item in copy.copy(lintables):
+            if item.path.is_dir():
+                for filename in all_files:
+                    if filename.startswith(str(item.path)):
+                        lintables.add(Lintable(filename))

test/TestUtils.py

@@ -213,9 +213,9 @@ def test_expand_paths_vars(test_path, expected, monkeypatch):
     ('reset_env_var', 'message_prefix'),
     (
         ('PATH', "Failed to locate command: "),
-        ('GIT_DIR', "Failed to discover yaml files to lint using git: "),
+        ('GIT_DIR', "Discovering files to lint: "),
     ),
-    ids=('no Git installed', 'outside Git repository'),
+    ids=('no-git-cli', 'outside-git-repo'),
 )
 def test_get_yaml_files_git_verbose(reset_env_var, message_prefix, monkeypatch, caplog):
     """Ensure that autodiscovery lookup failures are logged."""