Instrument handle_process_output test

To try to find the bug that causes it to fail on Cygwin on CI, but
not on other systems on CI, and not locally on Cygwin.

It looks like there's an extra line being read from stderr when the
test fails, so let's examine the lines themselves.

test/test_git.py

@@ -762,14 +762,17 @@ def test_environment(self, rw_dir):
     def test_handle_process_output(self):
         from git.cmd import handle_process_output, safer_popen
 
-        line_count = 5002
-        count = [None, 0, 0]
+        expected_line_count = 5002
+        line_counts = [None, 0, 0]
+        lines = [None, [], []]
 
-        def counter_stdout(line):
-            count[1] += 1
+        def stdout_handler(line):
+            line_counts[1] += 1
+            lines[1].append(line)
 
-        def counter_stderr(line):
-            count[2] += 1
+        def stderr_handler(line):
+            line_counts[2] += 1
+            lines[2].append(line)
 
         cmdline = [
             sys.executable,
@@ -784,10 +787,10 @@ def counter_stderr(line):
             shell=False,
         )
 
-        handle_process_output(proc, counter_stdout, counter_stderr, finalize_process)
+        handle_process_output(proc, stdout_handler, stderr_handler, finalize_process)
 
-        self.assertEqual(count[1], line_count)
-        self.assertEqual(count[2], line_count)
+        self.assertEqual(line_counts[1], expected_line_count, repr(lines[1]))
+        self.assertEqual(line_counts[2], expected_line_count, repr(lines[2]))
 
     def test_execute_kwargs_set_agrees_with_method(self):
         parameter_names = inspect.signature(cmd.Git.execute).parameters.keys()