run-task: clean up subprocess usage in _clean_git_checkout

This was copied from `run_command` in
8d7d8b1, but some stuff doesn't make
sense here:
- we don't stream the output to our log so we don't need to disable
  buffering or go through a TextIOWrapper
- we don't want to pass our stdin to the command

Instead we can use subprocess.run and let it capture stdout and check
the returncode.

Author: jcristau

src/taskgraph/run-task/run-task

@@ -597,26 +597,18 @@ def _clean_git_checkout(destination_path):
         "-nxdff",
     ]
     print_line(b"vcs", b"executing %r\n" % args)
-    p = subprocess.Popen(
+    p = subprocess.run(
         args,
-        # Disable buffering because we want to receive output
-        # as it is generated so timestamps in logs are
-        # accurate.
-        bufsize=0,
         stdout=subprocess.PIPE,
-        stdin=sys.stdin.fileno(),
+        encoding="latin1",
         cwd=destination_path,
         env=os.environ,
+        check=True,
     )
-    stdout = io.TextIOWrapper(p.stdout, encoding="latin1")
-    ret = p.wait()
-    if ret:
-        sys.exit(ret)
-    data = stdout.read()
     prefix = "Would remove "
     filenames = [
         os.path.join(destination_path, line[len(prefix) :])
-        for line in data.splitlines()
+        for line in p.stdout.splitlines()
     ]
     print_line(b"vcs", b"removing %r\n" % filenames)
     for filename in filenames:

test/test_scripts_run_task.py

@@ -1,5 +1,4 @@
 import functools
-import io
 import os
 import site
 import stat
@@ -293,23 +292,20 @@ def test_clean_git_checkout(monkeypatch, mock_stdin, run_task_mod):
     output_str = (
         f"{prefix}{untracked_dir_rel_path}/\n{prefix}{untracked_file_rel_path}\n"
     )
-    output_bytes = output_str.encode("latin1")
-    output = io.BytesIO(output_bytes)
+
+    real_popen = subprocess.Popen
 
     def _Popen(
         args,
-        bufsize=None,
-        stdout=None,
-        stderr=None,
-        stdin=None,
-        cwd=None,
-        env=None,
         **kwargs,
     ):
-        return Mock(
-            stdout=output,
-            wait=lambda: 0,
+        kwargs["stdin"] = subprocess.PIPE
+        proc = real_popen(
+            ["cat"],
+            **kwargs,
         )
+        proc.stdin.write(output_str)
+        return proc
 
     monkeypatch.setattr(
         subprocess,