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 b6211b89eb3 (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>
diff --git a/t/t6400-merge-df.sh b/t/t6400-merge-df.sh
index 400a4cd..f1b8461 100755
--- a/t/t6400-merge-df.sh
+++ b/t/t6400-merge-df.sh
@@ -124,7 +124,7 @@
 		git add . &&
 		git commit -m initial &&
 
-		git branch main &&
+		git branch topic &&
 		git branch other &&
 
 		git checkout other &&
@@ -132,10 +132,10 @@
 		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 &&