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

@@ -1,3 +1,4 @@
+import subprocess
 from pathlib import Path
 
 from . import hg
@@ -7,7 +8,14 @@ def identify_vcs(directory: Path):
     directory = directory.resolve()
     for p in [directory] + list(directory.parents):
         if (p / '.git').is_dir():
-            return git
+            check_ignore = subprocess.run(
+                ['git', 'check-ignore', '.'],
+                cwd=str(directory),
+                stderr=subprocess.DEVNULL,
+                stdout=subprocess.DEVNULL,
+            ).returncode
+            if check_ignore != 0:
+                return git
         if (p / '.hg').is_dir():
             return hg