t2106: make test independent of the current main branch name

We do have this wonderful shortcut `git checkout -` to go back to the
previous branch, thanks to the reflog.

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

t/t2106-update-index-assume-unchanged.sh

@@ -20,7 +20,7 @@ test_expect_success 'do not switch branches with dirty file' '
 	git checkout other &&
 	echo dirt >file &&
 	git update-index --assume-unchanged file &&
-	test_must_fail git checkout master
+	test_must_fail git checkout -
 '
 
 test_done