rebase: fix fork-point with zero arguments

When no arguments are specified, $switch_to is empty so we end up
passing the empty string to "git merge-base --fork-point", which causes
an error.  git-rebase carries on at this point, but in fact we have
failed to apply the fork-point operation.

It turns out that the test in t3400 that was meant to test this didn't
actually need the fork-point behaviour, so enhance it to make sure that
the fork-point is applied correctly.  The modified test fails without
the change to git-rebase.sh in this patch.

Reported-by: Andreas Krey <a.krey@gmx.de>
Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase.sh

@@ -534,7 +534,8 @@ esac
 
 if test "$fork_point" = t
 then
-	new_upstream=$(git merge-base --fork-point "$upstream_name" "$switch_to")
+	new_upstream=$(git merge-base --fork-point "$upstream_name" \
+			"${switch_to:-HEAD}")
 	if test -n "$new_upstream"
 	then
 		upstream=$new_upstream

t/t3400-rebase.sh

@@ -135,11 +135,19 @@ test_expect_success 'fail when upstream arg is missing and not configured' '
 '
 
 test_expect_success 'default to common base in @{upstream}s reflog if no upstream arg' '
+	git checkout -b default-base master &&
 	git checkout -b default topic &&
 	git config branch.default.remote . &&
-	git config branch.default.merge refs/heads/master &&
+	git config branch.default.merge refs/heads/default-base &&
 	git rebase &&
-	git rev-parse --verify master >expect &&
+	git rev-parse --verify default-base >expect &&
+	git rev-parse default~1 >actual &&
+	test_cmp expect actual &&
+	git checkout default-base &&
+	git reset --hard HEAD^ &&
+	git checkout default &&
+	git rebase &&
+	git rev-parse --verify default-base >expect &&
 	git rev-parse default~1 >actual &&
 	test_cmp expect actual
 '