fix: reject unsafe clone templates

Treat git clone --template as unsafe because caller-controlled
templates can install hooks that execute during clone.

Add regression coverage for both direct option and keyword forms.

References GHSA-6p8h-3wgx-97gf.
Validated against Git baseline a23bace9.

Author: codex

git/repo/base.py

@@ -149,6 +149,8 @@ class Repo:
         # Can override configuration variables that execute arbitrary commands:
         "--config",
         "-c",
+        # Can install hooks that execute during clone:
+        "--template",
     ]
     """Options to :manpage:`git-clone(1)` that allow arbitrary commands to be executed.
 
@@ -159,6 +161,9 @@ class Repo:
     The ``--config``/``-c`` option allows users to override configuration variables like
     ``protocol.allow`` and ``core.gitProxy`` to execute arbitrary commands:
     https://git-scm.com/docs/git-clone#Documentation/git-clone.txt---configltkeygtltvaluegt
+
+    The ``--template`` option can install hooks that execute during clone:
+    https://git-scm.com/docs/git-clone#Documentation/git-clone.txt---templatetemplate-directory
     """
 
     unsafe_git_archive_options = [

test/test_clone.py

@@ -128,6 +128,7 @@ def test_clone_unsafe_options(self, rw_repo):
                 "-c protocol.ext.allow=always",
                 "-cprotocol.ext.allow=always",
                 "-vcprotocol.ext.allow=always",
+                f"--template={tmp_dir}",
             ]
             for unsafe_option in unsafe_options:
                 with self.assertRaises(UnsafeOptionError):
@@ -142,6 +143,7 @@ def test_clone_unsafe_options(self, rw_repo):
                 {"config": "protocol.ext.allow=always"},
                 {"conf": "protocol.ext.allow=always"},
                 {"c": "protocol.ext.allow=always"},
+                {"template": tmp_dir},
             ]
             for unsafe_option in unsafe_options:
                 with self.assertRaises(UnsafeOptionError):