test: fix for latest versions of git

Since git 2.34 the big push with force test is broken thanks to:

726a228dfb (fast-export: fix surprising behavior with --first-parent, 2021-12-16)

Since this only affects pushing multiple heads to the same branch, it
probably is not that important.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>

test/main.t

@@ -682,7 +682,7 @@ test_expect_success 'remote big push force' '
 	EOF
 	) &&
 
-	check_branch hgrepo default six &&
+	check_branch hgrepo default four &&
 	check_branch hgrepo good_branch eight &&
 	check_branch hgrepo bad_branch nine &&
 	check_branch hgrepo new_branch ten &&