Fix Windows env var upcasing regression

This uses a simple hand-rolled context manager to patch the
NoDefaultCurrentDirectoryInExePath variable, instead of
unittest.mock.patch.dict. The latter set unrelated environment
variables to the original (same) values via os.environ, and as a
result, their names were all converted to upper-case on Windows.

Because only environment variables that are actually set through
os.environ have their names upcased, the only variable whose name
should be upcased now is NoDefaultCurrentDirectoryInExePath, which
should be fine (it has a single established use/meaning in Windows,
where it's treated case-insensitively as environment variables in
Windows *usually* are).

git/cmd.py

@@ -14,7 +14,6 @@
 import subprocess
 import threading
 from textwrap import dedent
-import unittest.mock
 
 from git.compat import (
     defenc,
@@ -24,7 +23,7 @@
     is_win,
 )
 from git.exc import CommandError
-from git.util import is_cygwin_git, cygpath, expand_path, remove_password_if_present
+from git.util import is_cygwin_git, cygpath, expand_path, remove_password_if_present, patch_env
 
 from .exc import GitCommandError, GitCommandNotFound, UnsafeOptionError, UnsafeProtocolError
 from .util import (
@@ -965,10 +964,10 @@ def execute(
                     '"kill_after_timeout" feature is not supported on Windows.',
                 )
             # Only search PATH, not CWD. This must be in the *caller* environment. The "1" can be any value.
-            patch_caller_env = unittest.mock.patch.dict(os.environ, {"NoDefaultCurrentDirectoryInExePath": "1"})
+            maybe_patch_caller_env = patch_env("NoDefaultCurrentDirectoryInExePath", "1")
         else:
             cmd_not_found_exception = FileNotFoundError  # NOQA # exists, flake8 unknown @UndefinedVariable
-            patch_caller_env = contextlib.nullcontext()
+            maybe_patch_caller_env = contextlib.nullcontext()
         # end handle
 
         stdout_sink = PIPE if with_stdout else getattr(subprocess, "DEVNULL", None) or open(os.devnull, "wb")
@@ -984,7 +983,7 @@ def execute(
             istream_ok,
         )
         try:
-            with patch_caller_env:
+            with maybe_patch_caller_env:
                 proc = Popen(
                     command,
                     env=env,

git/util.py

@@ -158,6 +158,20 @@ def cwd(new_dir: PathLike) -> Generator[PathLike, None, None]:
         os.chdir(old_dir)
 
 
+@contextlib.contextmanager
+def patch_env(name: str, value: str) -> Generator[None, None, None]:
+    """Context manager to temporarily patch an environment variable."""
+    old_value = os.getenv(name)
+    os.environ[name] = value
+    try:
+        yield
+    finally:
+        if old_value is None:
+            del os.environ[name]
+        else:
+            os.environ[name] = old_value
+
+
 def rmtree(path: PathLike) -> None:
     """Remove the given recursively.
 
@@ -935,7 +949,7 @@ def _obtain_lock_or_raise(self) -> None:
             )
 
         try:
-            with open(lock_file, mode='w'):
+            with open(lock_file, mode="w"):
                 pass
         except OSError as e:
             raise IOError(str(e)) from e