Log stdin arg as such, and test that this is done

This changes how the Popen call debug logging line shows the
informal summary of what kind of thing is being passed as the stdin
argument to Popen, showing it with stdin= rather than istream=.

The new test, with "istream" in place of "stdin", passed before the
code change in the git.cmd module, failed once "istream" was
changed to "stdin" in the test, and then, as expected, passed again
once "istream=" was changed to "stdin=" in the log.debug call in
git.cmd.Git.execute.

Author: EliahKagan

Diff for: git/cmd.py

@@ -979,7 +979,7 @@ def execute(
         if shell is None:
             shell = self.USE_SHELL
         log.debug(
-            "Popen(%s, cwd=%s, universal_newlines=%s, shell=%s, istream=%s)",
+            "Popen(%s, cwd=%s, universal_newlines=%s, shell=%s, stdin=%s)",
             redacted_command,
             cwd,
             universal_newlines,

Diff for: test/test_git.py

@@ -124,6 +124,16 @@ def test_it_logs_if_it_uses_a_shell(self, case):
             mock_popen = self._do_shell_combo(value_in_call, value_from_class)
         self._assert_logged_for_popen(log_watcher, "shell", mock_popen.call_args.kwargs["shell"])
 
+    @ddt.data(
+        ("None", None),
+        ("<valid stream>", subprocess.PIPE),
+    )
+    def test_it_logs_istream_summary_for_stdin(self, case):
+        expected_summary, istream_argument = case
+        with self.assertLogs(cmd.log, level=logging.DEBUG) as log_watcher:
+            self.git.execute(["git", "version"], istream=istream_argument)
+        self._assert_logged_for_popen(log_watcher, "stdin", expected_summary)
+
     def test_it_executes_git_and_returns_result(self):
         self.assertRegex(self.git.execute(["git", "version"]), r"^git version [\d\.]{2}.*$")