git: Add option for no_config in git shell

This causes git commands to run without considering any
system or user configuration. In some cases it's easier to
ban all overrides rather than have to override things on
a flag by flag basis.

Topic: gitconfig

revup/git.py

@@ -269,19 +269,33 @@ def get_scratch_dir(self) -> str:
         """
         return f"{self.repo_root}/.revup" if self.keep_temp else self.temp_dir.name
 
-    async def git(self, *args: str, **kwargs: Any) -> Tuple[int, str]:
+    async def git(
+        self,
+        *args: str,
+        no_config: bool = False,
+        env: Optional[Dict[str, str]] = None,
+        **kwargs: Any,
+    ) -> Tuple[int, str]:
         """
         Run a git command.  The returned stdout has trailing newlines stripped.
 
         Args:
             *args: Arguments to git
             **kwargs: Any valid kwargs for sh()
         """
+        git_env = {}
+        if no_config:
+            git_env.update({
+                "HOME": "/dev/null",
+                "GIT_CONFIG_NOSYSTEM": "1",
+            })
+        if env is not None:
+            git_env.update(env)
 
         def _maybe_rstrip(s: Tuple[int, str]) -> Tuple[int, str]:
             return (s[0], s[1].rstrip())
 
-        return _maybe_rstrip(await self.sh.sh(*((self.git_path,) + args), **kwargs))
+        return _maybe_rstrip(await self.sh.sh(*((self.git_path,) + args), env=git_env, **kwargs))
 
     async def git_return_code(self, *args: str, **kwargs: Any) -> int:
         return (await self.git(raiseonerror=False, *args, **kwargs))[0]