tests: avoid using the branch name `main`

In the near future, we want to change Git's default branch name to
`main`. In preparation for that, stop using it as a branch name in the
test suite. Replace that branch name by `topic`, the same name we used
to rename variations of `master` in b6211b89eb (tests: avoid variations
of the `master` branch name, 2020-09-26).

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

t/t6012-rev-list-simplify.sh

@@ -171,7 +171,7 @@ test_expect_success '--full-diff is not affected by --parents' '
 test_expect_success 'rebuild repo' '
 	rm -rf .git * &&
 	git init &&
-	git switch -c main &&
+	git switch -c topic &&
 
 	echo base >file &&
 	git add file &&
@@ -186,7 +186,7 @@ test_expect_success 'rebuild repo' '
 	git add file &&
 	test_commit B &&
 
-	git switch main &&
+	git switch topic &&
 	test_must_fail git merge -m "M" B &&
 	echo A >file &&
 	echo B >>file &&
@@ -207,7 +207,7 @@ test_expect_success 'rebuild repo' '
 	git merge -m R -Xtheirs X &&
 	note R &&
 
-	git switch main &&
+	git switch topic &&
 	git merge -m N R &&
 	note N &&
 
@@ -221,7 +221,7 @@ test_expect_success 'rebuild repo' '
 	git add z &&
 	test_commit Z &&
 
-	git switch main &&
+	git switch topic &&
 	git merge -m O Z &&
 	note O &&
 

t/t6400-merge-df.sh

@@ -124,7 +124,7 @@ test_expect_success 'Simple merge in repo with interesting pathnames' '
 		git add . &&
 		git commit -m initial &&
 
-		git branch main &&
+		git branch topic &&
 		git branch other &&
 
 		git checkout other &&
@@ -132,10 +132,10 @@ test_expect_success 'Simple merge in repo with interesting pathnames' '
 		git add -u &&
 		git commit -m other &&
 
-		git checkout main &&
-		echo main >foo/bar/baz &&
+		git checkout topic &&
+		echo topic >foo/bar/baz &&
 		git add -u &&
-		git commit -m main &&
+		git commit -m topic &&
 
 		git merge other &&
 		git ls-files -s >out &&

t/t6409-merge-subtree.sh

@@ -35,11 +35,11 @@ test_expect_success 'setup branch sub' '
 	test_commit foo
 '
 
-test_expect_success 'setup branch main' '
-	git checkout -b main master &&
+test_expect_success 'setup topic branch' '
+	git checkout -b topic master &&
 	git merge -s ours --no-commit --allow-unrelated-histories sub &&
 	git read-tree --prefix=dir/ -u sub &&
-	git commit -m "initial merge of sub into main" &&
+	git commit -m "initial merge of sub into topic" &&
 	test_path_is_file dir/foo.t &&
 	test_path_is_file hello
 '
@@ -49,9 +49,9 @@ test_expect_success 'update branch sub' '
 	test_commit bar
 '
 
-test_expect_success 'update branch main' '
-	git checkout main &&
-	git merge -s subtree sub -m "second merge of sub into main" &&
+test_expect_success 'update topic branch' '
+	git checkout topic &&
+	git merge -s subtree sub -m "second merge of sub into topic" &&
 	test_path_is_file dir/bar.t &&
 	test_path_is_file dir/foo.t &&
 	test_path_is_file hello

t/t6430-merge-recursive.sh

@@ -663,7 +663,7 @@ test_expect_failure 'merge-recursive rename vs. rename/symlink' '
 
 test_expect_success 'merging with triple rename across D/F conflict' '
 	git reset --hard HEAD &&
-	git checkout -b main &&
+	git checkout -b topic &&
 	git rm -rf . &&
 
 	echo "just a file" >sub1 &&
@@ -682,7 +682,7 @@ test_expect_success 'merging with triple rename across D/F conflict' '
 	test_tick &&
 	git commit -a -m changesimplefile &&
 
-	git checkout main &&
+	git checkout topic &&
 	git rm sub1 &&
 	git mv sub2 sub1 &&
 	test_tick &&