Add guard against the hooks directory being a symbolic link

`pre-commit install` shouldn't mess with directories
residing outside of the git repository.

pre_commit/commands/install_uninstall.py

@@ -70,6 +70,22 @@ def _install_hook_script(
 ) -> None:
     hook_path, legacy_path = _hook_paths(hook_type, git_dir=git_dir)
 
+    # If the hooks directory is a symlink we need to be careful
+    if os.path.islink(os.path.dirname(hook_path)):
+        git_dir = git_dir if git_dir is not None else git.get_git_common_dir()
+        # If the hooks directory links to a directory outside the
+        # git repo we shouldn't try to mess with it
+        if os.path.commonpath(
+            [os.path.realpath(git_dir), os.path.realpath(hook_path)]
+            ) != os.path.realpath(git_dir):
+            logger.error(
+                'Cowardly refusing to install hook script to a directory '
+                'outside of the git repo.\n'
+                f'hint: {os.path.dirname(hook_path)} is a symbolic link '
+                f'to {os.path.realpath(os.path.dirname(hook_path))}.'
+            )
+            return 1
+
     os.makedirs(os.path.dirname(hook_path), exist_ok=True)
 
     # If we have an existing hook, move it to pre-commit.legacy
@@ -109,6 +125,7 @@ def _install_hook_script(
     make_executable(hook_path)
 
     output.write_line(f'pre-commit installed at {hook_path}')
+    return 0
 
 
 def install(
@@ -128,12 +145,14 @@ def install(
         return 1
 
     for hook_type in _hook_types(config_file, hook_types):
-        _install_hook_script(
+        ret = _install_hook_script(
             config_file, hook_type,
             overwrite=overwrite,
             skip_on_missing_config=skip_on_missing_config,
             git_dir=git_dir,
         )
+        if ret != 0:
+            return ret
 
     if hooks:
         install_hooks(config_file, store)

tests/commands/install_uninstall_test.py

@@ -109,6 +109,14 @@ def test_install_hooks_dead_symlink(in_git_dir, store):
     assert hook.exists()
 
 
+def test_install_hooks_symlink_outisde_git_repo(in_git_dir, store):
+    hook_dir = in_git_dir.join('.git/hooks')
+    hook_dir.mksymlinkto(in_git_dir.join("../hooks"))
+    hook = hook_dir.join('pre-commit')
+    assert install(C.CONFIG_FILE, store, hook_types=['pre-commit']) != 0
+    assert not hook.exists()
+
+
 def test_uninstall_does_not_blow_up_when_not_there(in_git_dir):
     assert uninstall(C.CONFIG_FILE, hook_types=['pre-commit']) == 0