Eliminate istream_ok variable

In Git.execute, the stdin argument to Popen is the only one where a
compound expression (rather than a single term) is currently
passed. So having that be the same in the log message makes it
easier to understand what is going on, as well as to see how the
information shown in the log corresponds to what Popen receives.

git/cmd.py

@@ -973,16 +973,13 @@ def execute(
         # end handle
 
         stdout_sink = PIPE if with_stdout else getattr(subprocess, "DEVNULL", None) or open(os.devnull, "wb")
-        istream_ok = "None"
-        if istream:
-            istream_ok = "<valid stream>"
         if shell is None:
             shell = self.USE_SHELL
         log.debug(
             "Popen(%s, cwd=%s, stdin=%s, shell=%s, universal_newlines=%s)",
             redacted_command,
             cwd,
-            istream_ok,
+            "<valid stream>" if istream else "None",
             shell,
             universal_newlines,
         )