Extract env building into a testable helper.

Previously env dict building was untested and mixed with other mutative
actions. Extract the dict building into a dedicated function and author
tests to ensure the functionality is working as expected.

BUG: b/255376186
BUG: https://crbug.com/gerrit/16247
Change-Id: I0c88e53eb285c5c3fb27f8e6b3a903aedb8e02a8
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/351874
Reviewed-by: LaMont Jones <lamontjones@google.com>
Tested-by: Sam Saccone <samccone@google.com>

tests/test_git_command.py

@@ -15,6 +15,7 @@
 """Unittests for the git_command.py module."""
 
 import re
+import os
 import unittest
 
 try:
@@ -26,6 +27,38 @@ import git_command
 import wrapper
 
 
+class GitCommandTest(unittest.TestCase):
+  """Tests the GitCommand class (via git_command.git)."""
+
+  def setUp(self):
+
+    def realpath_mock(val):
+      return val
+
+    mock.patch.object(os.path, 'realpath', side_effect=realpath_mock).start()
+
+  def tearDown(self):
+    mock.patch.stopall()
+
+  def test_alternative_setting_when_matching(self):
+    r = git_command._build_env(
+      objdir = 'zap/objects',
+      gitdir = 'zap'
+    )
+
+    self.assertIsNone(r.get('GIT_ALTERNATE_OBJECT_DIRECTORIES'))
+    self.assertEqual(r.get('GIT_OBJECT_DIRECTORY'),  'zap/objects')
+
+  def test_alternative_setting_when_different(self):
+    r = git_command._build_env(
+      objdir = 'wow/objects',
+      gitdir = 'zap'
+    )
+
+    self.assertEqual(r.get('GIT_ALTERNATE_OBJECT_DIRECTORIES'), 'zap/objects')
+    self.assertEqual(r.get('GIT_OBJECT_DIRECTORY'),  'wow/objects')
+
+
 class GitCallUnitTest(unittest.TestCase):
   """Tests the _GitCall class (via git_command.git)."""