Use minimal environment in Mercurial loader

This generalizes the work done in ef3a2ba7 to (supposedly) all places invoking Mercurial. In short, this limits the environment to the smallest subset needed (i.e. $PATH) and uses the Mercurial-specific variables to disable user customizations and configs.

---

Migrated from D5146 (view on Phabricator)

swh/loader/mercurial/identify.py

-# Copyright (C) 2020  The Software Heritage developers
+# Copyright (C) 2020-2021  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
 from codecs import escape_decode  # type: ignore
 import json
-import os
 from pathlib import Path
 import re
 import subprocess
@@ -15,6 +14,7 @@ from typing import Any, Dict, Iterator, List, NamedTuple, Optional, Union
 # control
 import click
 
+from swh.loader.mercurial.utils import get_minimum_env
 from swh.model.cli import identify_object
 from swh.model.hashutil import hash_to_bytehex
 from swh.model.identifiers import normalize_timestamp, swhid
@@ -198,7 +198,7 @@ class Hg:
     def _output(self, *args) -> bytes:
         """Return the outpout of a `hg` call."""
         return subprocess.check_output(
-            ["hg", *args], cwd=self._root, env=self._get_env()
+            ["hg", *args], cwd=self._root, env=get_minimum_env()
         )
 
     def _call(self, *args) -> None:
@@ -208,18 +208,9 @@ class Hg:
             cwd=self._root,
             stderr=subprocess.PIPE,
             stdout=subprocess.PIPE,
-            env=self._get_env(),
+            env=get_minimum_env(),
         )
 
-    def _get_env(self) -> Dict[str, str]:
-        """Return the smallest viable environment for `hg` suprocesses"""
-        env = {
-            "PATH": os.environ["PATH"],
-            "HGPLAIN": "",  # Tells Mercurial to disable output customization
-            "HGRCPATH": "",  # Tells Mercurial to ignore config files
-        }
-        return env
-
     def root(self) -> Path:
         """Return the root of the Mercurial repository."""
         return self._root