Merge pull request #1727 from mrogaski/1610_git_toplevel_relpath

git: changed rev-parse option for Git 2.25 changes to --show-toplevel

pre_commit/git.py

@@ -47,21 +47,26 @@ def no_git_env(
 
 
 def get_root() -> str:
+    # Git 2.25 introduced a change to "rev-parse --show-toplevel" that exposed
+    # underlying volumes for Windows drives mapped with SUBST.  We use
+    # "rev-parse --show-cdup" to get the appropriate path, but must perform
+    # an extra check to see if we are in the .git directory.
     try:
-        root = cmd_output('git', 'rev-parse', '--show-toplevel')[1].strip()
+        root = os.path.realpath(
+            cmd_output('git', 'rev-parse', '--show-cdup')[1].strip(),
+        )
+        git_dir = os.path.realpath(get_git_dir())
     except CalledProcessError:
         raise FatalError(
             'git failed. Is it installed, and are you in a Git repository '
             'directory?',
         )
-    else:
-        if root == '':  # pragma: no cover (old git)
-            raise FatalError(
-                'git toplevel unexpectedly empty! make sure you are not '
-                'inside the `.git` directory of your repository.',
-            )
-        else:
-            return root
+    if os.path.commonpath((root, git_dir)) == git_dir:
+        raise FatalError(
+            'git toplevel unexpectedly empty! make sure you are not '
+            'inside the `.git` directory of your repository.',
+        )
+    return root
 
 
 def get_git_dir(git_root: str = '.') -> str: