remove master references from tests

tests/check_merge_conflict_test.py

diff --git a/tests/check_merge_conflict_test.py b/tests/check_merge_conflict_test.py
index 76c4283..64112d7 100644
--- a/tests/check_merge_conflict_test.py
+++ b/tests/check_merge_conflict_test.py
@@ -27,10 +27,10 @@
 
     cmd_output('git', 'clone', str(repo1), str(repo2))
 
-    # Commit in master
+    # Commit in mainline
     with repo1.as_cwd():
         repo1_f1.write('parent\n')
-        git_commit('-am', 'master commit2')
+        git_commit('-am', 'mainline commit2')
 
     # Commit in clone and pull
     with repo2.as_cwd():
@@ -82,10 +82,10 @@
 
     cmd_output('git', 'clone', str(repo1), str(repo2))
 
-    # Commit in master
+    # Commit in mainline
     with repo1.as_cwd():
         repo1_f1.write('parent\n')
-        git_commit('-am', 'master commit2')
+        git_commit('-am', 'mainline commit2')
 
     # Commit in clone and pull without committing
     with repo2.as_cwd():
@@ -112,7 +112,7 @@
 
 
 @pytest.mark.parametrize(
-    'contents', (b'<<<<<<< HEAD\n', b'=======\n', b'>>>>>>> master\n'),
+    'contents', (b'<<<<<<< HEAD\n', b'=======\n', b'>>>>>>> main\n'),
 )
 def test_merge_conflicts_failing(contents, repository_pending_merge):
     repository_pending_merge.join('f2').write_binary(contents)
@@ -150,7 +150,7 @@
     cmd_output('git', 'worktree', 'add', str(worktree))
     with worktree.as_cwd():
         cmd_output(
-            'git', 'pull', '--no-rebase', 'origin', 'master', retcode=None,
+            'git', 'pull', '--no-rebase', 'origin', 'HEAD', retcode=None,
         )
         msg = f1_is_a_conflict_file.join('.git/worktrees/worktree/MERGE_MSG')
         assert msg.exists()

tests/check_vcs_permalinks_test.py

diff --git a/tests/check_vcs_permalinks_test.py b/tests/check_vcs_permalinks_test.py
index 01ce94d..324b70c 100644
--- a/tests/check_vcs_permalinks_test.py
+++ b/tests/check_vcs_permalinks_test.py
@@ -16,9 +16,9 @@
         # tags are ok
         b'https://github.com/asottile/test/blob/1.0.0/foo%20bar#L1\n'
         # links to files but not line numbers are ok
-        b'https://github.com/asottile/test/blob/master/foo%20bar\n'
+        b'https://github.com/asottile/test/blob/main/foo%20bar\n'
         # regression test for overly-greedy regex
-        b'https://github.com/ yes / no ? /blob/master/foo#L1\n',
+        b'https://github.com/ yes / no ? /blob/main/foo#L1\n',
     )
     assert not main((str(f),))
 
@@ -26,17 +26,15 @@
 def test_failing(tmpdir, capsys):
     with tmpdir.as_cwd():
         tmpdir.join('f.txt').write_binary(
-            b'https://github.com/asottile/test/blob/master/foo#L1\n'
-            b'https://example.com/asottile/test/blob/master/foo#L1\n'
+            b'https://github.com/asottile/test/blob/main/foo#L1\n'
             b'https://example.com/asottile/test/blob/main/foo#L1\n',
         )
 
         assert main(('f.txt', '--additional-github-domain', 'example.com'))
         out, _ = capsys.readouterr()
         assert out == (
-            'f.txt:1:https://github.com/asottile/test/blob/master/foo#L1\n'
-            'f.txt:2:https://example.com/asottile/test/blob/master/foo#L1\n'
-            'f.txt:3:https://example.com/asottile/test/blob/main/foo#L1\n'
+            'f.txt:1:https://github.com/asottile/test/blob/main/foo#L1\n'
+            'f.txt:2:https://example.com/asottile/test/blob/main/foo#L1\n'
             '\n'
             'Non-permanent github link detected.\n'
             'On any page on github press [y] to load a permalink.\n'

tests/no_commit_to_branch_test.py

diff --git a/tests/no_commit_to_branch_test.py b/tests/no_commit_to_branch_test.py
index eaae5e6..7d37e49 100644
--- a/tests/no_commit_to_branch_test.py
+++ b/tests/no_commit_to_branch_test.py
@@ -11,13 +11,13 @@
 def test_other_branch(temp_git_dir):
     with temp_git_dir.as_cwd():
         cmd_output('git', 'checkout', '-b', 'anotherbranch')
-        assert is_on_branch({'master'}) is False
+        assert is_on_branch({'placeholder'}) is False
 
 
 def test_multi_branch(temp_git_dir):
     with temp_git_dir.as_cwd():
         cmd_output('git', 'checkout', '-b', 'another/branch')
-        assert is_on_branch({'master'}) is False
+        assert is_on_branch({'placeholder'}) is False
 
 
 def test_multi_branch_fail(temp_git_dir):
@@ -26,9 +26,10 @@
         assert is_on_branch({'another/branch'}) is True
 
 
-def test_master_branch(temp_git_dir):
+def test_exact_branch(temp_git_dir):
     with temp_git_dir.as_cwd():
-        assert is_on_branch({'master'}) is True
+        cmd_output('git', 'checkout', '-b', 'branchname')
+        assert is_on_branch({'branchname'}) is True
 
 
 def test_main_branch_call(temp_git_dir):
@@ -50,11 +51,11 @@
         assert is_on_branch(set(), {'another/.*'}) is True
 
 
-@pytest.mark.parametrize('branch_name', ('master', 'another/branch'))
+@pytest.mark.parametrize('branch_name', ('somebranch', 'another/branch'))
 def test_branch_pattern_multiple_branches_fail(temp_git_dir, branch_name):
     with temp_git_dir.as_cwd():
         cmd_output('git', 'checkout', '-b', branch_name)
-        assert main(('--branch', 'master', '--pattern', 'another/.*'))
+        assert main(('--branch', 'somebranch', '--pattern', 'another/.*'))
 
 
 def test_main_default_call(temp_git_dir):