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>

git-rebase--interactive.sh

@@ -360,17 +360,15 @@ do_next () {
 		pick_one -n $sha1 || failed=t
 		case "$(peek_next_command)" in
 		squash|s)
-			EDIT_COMMIT=
 			USE_OUTPUT=output
 			MSG_OPT=-F
-			MSG_FILE="$MSG"
+			EDIT_OR_FILE="$MSG"
 			cp "$MSG" "$SQUASH_MSG"
 			;;
 		*)
-			EDIT_COMMIT=-e
 			USE_OUTPUT=
 			MSG_OPT=
-			MSG_FILE=
+			EDIT_OR_FILE=-e
 			rm -f "$SQUASH_MSG" || exit
 			cp "$MSG" "$GIT_DIR"/SQUASH_MSG
 			rm -f "$GIT_DIR"/MERGE_MSG || exit
@@ -384,7 +382,8 @@ do_next () {
 			GIT_AUTHOR_NAME="$GIT_AUTHOR_NAME" \
 			GIT_AUTHOR_EMAIL="$GIT_AUTHOR_EMAIL" \
 			GIT_AUTHOR_DATE="$GIT_AUTHOR_DATE" \
-			$USE_OUTPUT git commit --no-verify $MSG_OPT "$MSG_FILE" $EDIT_COMMIT || failed=t
+			$USE_OUTPUT git commit --no-verify \
+				$MSG_OPT "$EDIT_OR_FILE" || failed=t
 		fi
 		if test $failed = t
 		then

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
 '