diff --git a/pre_commit/main.py b/pre_commit/main.py
index c195dabaa..a8886b6ed 100644
--- a/pre_commit/main.py
+++ b/pre_commit/main.py
@@ -137,9 +137,11 @@ def _add_run_options(parser: argparse.ArgumentParser) -> None:
         ),
     )
     parser.add_argument(
-        '--is-squash-merge',
+        '--squash-merge',
         action='store_const',
         const='1',
+        default='0',
+        dest='is_squash_merge',
         help=(
             'During a post-merge hook, indicates whether the merge was a '
             'squash merge'
diff --git a/tests/commands/hook_impl_test.py b/tests/commands/hook_impl_test.py
index fe2fec3c9..c38b9caa1 100644
--- a/tests/commands/hook_impl_test.py
+++ b/tests/commands/hook_impl_test.py
@@ -97,7 +97,7 @@ def call(*_, **__):
         ('pre-push', ['branch_name', 'remote_name']),
         ('commit-msg', ['.git/COMMIT_EDITMSG']),
         ('post-commit', []),
-        ('post-merge', [True]),
+        ('post-merge', ['1']),
         ('post-checkout', ['old_head', 'new_head', '1']),
         # multiple choices for commit-editmsg
         ('prepare-commit-msg', ['.git/COMMIT_EDITMSG']),
diff --git a/tests/commands/install_uninstall_test.py b/tests/commands/install_uninstall_test.py
index 49aa4d8a9..c7d392ab8 100644
--- a/tests/commands/install_uninstall_test.py
+++ b/tests/commands/install_uninstall_test.py
@@ -791,9 +791,12 @@ def test_post_merge_integration(tempdir_factory, store):
         cmd_output('git', 'add', '.')
         git_commit()
 
-        cmd_output('git', 'switch', 'master')
+        cmd_output('git', 'checkout', 'master')
         install(C.CONFIG_FILE, store, hook_types=['post-merge'])
-        retc, stdout, stderr = cmd_output('git', 'merge', 'branch')
+        retc, stdout, stderr = cmd_output_mocked_pre_commit_home(
+            'git', 'merge', 'branch',
+            tempdir_factory=tempdir_factory,
+        )
         assert retc == 0
         assert os.path.exists('post-merge.tmp')