t3404: use configured shell instead of /bin/sh

The fake-editor shell script invoked /bin/sh; normally this
is fine, unless the /bin/sh doesn't meet our compatibility
requirements, as is the case with Solaris. Specifically, the
$() syntax used by fake-editor is not understood.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t3404-rebase-interactive.sh

@@ -61,8 +61,8 @@ test_expect_success 'setup' '
 	git tag I
 '
 
-cat > fake-editor.sh <<\EOF
-#!/bin/sh
+echo "#!$SHELL" >fake-editor
+cat >> fake-editor.sh <<\EOF
 case "$1" in
 */COMMIT_EDITMSG)
 	test -z "$FAKE_COMMIT_MESSAGE" || echo "$FAKE_COMMIT_MESSAGE" > "$1"