t9902: switch default branch name to main

Remove the penultimate use of "GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=
master" in our test suite. We have slowly been removing these ever
since we started to switch the default branch name used in tests to
"main".

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: phillipwood

t/t9902-completion.sh

@@ -11,9 +11,9 @@ test_description='test bash completion'
 # untraceable with such ancient Bash versions.
 test_untraceable=UnfortunatelyYes
 
-# Override environment and always use master for the default initial branch
+# Override environment and always use main for the default initial branch
 # name for these tests, so that rev completion candidates are as expected.
-GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=master
+GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
 export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 
 . ./lib-bash.sh
@@ -1453,7 +1453,7 @@ test_expect_success 'git bisect - start subcommand arguments before double-dash
 		HEAD Z
 		final Z
 		initial Z
-		master Z
+		main Z
 		EOF
 	)
 '