t6[4-9]*: adjust the references to the default branch name "main"

This trick was performed via

	$ (cd t &&
	   sed -i -e 's/master/main/g' -e 's/MASTER/MAIN/g' \
		-e 's/Master/Main/g' -- t6[4-9]*.sh)

This allows us to define `GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main`
for those tests.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6432-merge-recursive-space-options.sh

@@ -2,16 +2,16 @@
 
 test_description='merge-recursive space options
 
-* [master] Clarify
+* [main] Clarify
  ! [remote] Remove cruft
 --
  + [remote] Remove cruft
-*  [master] Clarify
+*  [main] Clarify
 *+ [remote^] Initial revision
 *   ok 1: setup
 '
 
-GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=master
+GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
 export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 
 . ./test-lib.sh
@@ -90,7 +90,7 @@ test_expect_success 'setup' '
 	mv text.txt+ text.txt &&
 	git commit -a -m "Remove cruft" &&
 
-	git checkout master &&
+	git checkout main &&
 	sed -e "
 			s/\(not in his right mind\),\(.*\)/\1;\2Q/
 			s/Quite correct\(.*\)/It is too correct\1Q/