Fix submodule squashing into unrelated commit

Actually, I think the issue is pretty independent of submodules; when
"git commit" gets an empty parameter, it misinterprets it as a file.

So avoid passing an empty parameter to "git commit".

Actually, this is a nice cleanup, as MSG_FILE and EDIT_COMMIT were mutually
exclusive; use one variable instead

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

t/t3404-rebase-interactive.sh

@@ -484,7 +484,7 @@ test_expect_success 'submodule rebase setup' '
 	git commit -a -m "Three changes submodule"
 '
 
-test_expect_failure 'submodule rebase -i' '
+test_expect_success 'submodule rebase -i' '
 	FAKE_LINES="1 squash 2 3" git rebase -i A
 '