t3404: make tests more self-contained

As its very first action, t3404 installs (via set_fake_editor) a
specialized $EDITOR which simplifies automated 'rebase -i' testing. Many
tests rely upon this setting, thus tests which need a different editor
must take extra care upon completion to restore $EDITOR in order to
avoid breaking following tests. This places extra burden upon such tests
and requires that they undesirably have extra knowledge about
surrounding tests. Ease this burden by having each test install the
$EDITOR it requires, rather than relying upon a global setting.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>