test_logging_handlers: Mock out `git describe` because can be slow.

This cuts out about 11 calls to `git describe`.  In a nice fast LXC
container following our instructions for development on a Linux host,
this might save "only" about 1.5s; in a dev environment on a Windows
host, the savings have been clocked at 49s, presumably due to an
extremely slow filesystem in the VM.

The tests weren't doing much with this codepath as they were, and
there isn't a lot of value to be gained by testing it anyway; it's
totally non-critical and rarely changes.

[Commit message rewritten by greg.]

zerver/logging_handlers.py

@@ -18,7 +18,7 @@ from zerver.lib.queue import queue_json_publish
 from version import ZULIP_VERSION
 
 def try_git_describe() -> Optional[str]:
-    try:
+    try:  # nocoverage
         return subprocess.check_output(
             ['git',
              '--git-dir', os.path.join(os.path.dirname(__file__), '../.git'),

zerver/tests/test_logging_handlers.py

@@ -9,7 +9,7 @@ from django.http import HttpRequest, HttpResponse
 from django.test import RequestFactory, TestCase
 from django.utils.log import AdminEmailHandler
 from functools import wraps
-from mock import patch
+from mock import MagicMock, patch
 from mypy_extensions import NoReturn
 from typing import Any, Callable, Dict, Mapping, Optional, Text, Iterator
 
@@ -61,7 +61,9 @@ class AdminNotifyHandlerTest(ZulipTestCase):
             if isinstance(h, AdminNotifyHandler)
         ][0]
 
-    def test_basic(self) -> None:
+    @patch('zerver.logging_handlers.try_git_describe')
+    def test_basic(self, mock_function: MagicMock) -> None:
+        mock_function.return_value = None
         """A random exception passes happily through AdminNotifyHandler"""
         handler = self.get_admin_zulip_handler()
         try:
@@ -92,7 +94,9 @@ class AdminNotifyHandlerTest(ZulipTestCase):
             patched_notify.assert_called_once()
             return patched_notify.call_args[0][0]
 
-    def test_long_exception_request(self) -> None:
+    @patch('zerver.logging_handlers.try_git_describe')
+    def test_long_exception_request(self, mock_function: MagicMock) -> None:
+        mock_function.return_value = None
         """A request with no stack and multi-line report.getMessage() is handled properly"""
         record = self.simulate_error()
         record.exc_info = None
@@ -105,7 +109,9 @@ class AdminNotifyHandlerTest(ZulipTestCase):
         self.assertEqual(report['stack_trace'], 'message\nmoremesssage\nmore')
         self.assertEqual(report['message'], 'message')
 
-    def test_request(self) -> None:
+    @patch('zerver.logging_handlers.try_git_describe')
+    def test_request(self, mock_function: MagicMock) -> None:
+        mock_function.return_value = None
         """A normal request is handled properly"""
         record = self.simulate_error()