Prevent matching bogus parent git directories

If our current directory is not tracked by git we should assume
that the project does not use git as the vcs as any match will
be a false positive.

flit/vcs/__init__.py

@@ -1,12 +1,24 @@
+import subprocess
 from pathlib import Path
 
 from . import hg
 from . import git
 
+def git_validate_ignore(directory: Path) -> bool:
+    check_ignore = subprocess.run(
+        ['git', 'check-ignore', '.'],
+        cwd=str(directory),
+        stderr=subprocess.DEVNULL,
+        stdout=subprocess.DEVNULL,
+    ).returncode
+    if check_ignore == 0:
+        return False
+    return True
+
 def identify_vcs(directory: Path):
     directory = directory.resolve()
     for p in [directory] + list(directory.parents):
-        if (p / '.git').is_dir():
+        if (p / '.git').is_dir() and git_validate_ignore(directory):
             return git
         if (p / '.hg').is_dir():
             return hg

tests/test_sdist.py

@@ -1,6 +1,7 @@
 import ast
 from os.path import join as pjoin
 from pathlib import Path
+from unittest.mock import patch
 import pytest
 from shutil import which, copy, copytree
 import sys
@@ -81,7 +82,8 @@ def test_get_files_list_git(copy_sample):
 
     builder = sdist.SdistBuilder.from_ini_path(td / 'pyproject.toml')
     with MockCommand('git', LIST_FILES_GIT):
-        files = builder.select_files()
+        with patch('flit.vcs.git_validate_ignore', return_value=True):
+            files = builder.select_files()
 
     assert set(files) == {
         'foo', pjoin('dir1', 'bar'), pjoin('dir1', 'subdir', 'qux'),