"rebase -i": support special-purpose editor to edit insn sheet

"rebase -i": support special-purpose editor to edit insn sheet

The insn sheet used by "rebase -i" is designed to be easily editable by
any text editor, but an editor that is specifically meant for it (but
is otherwise unsuitable for editing regular text files) could be useful
by allowing drag & drop reordering in a GUI environment, for example.

The GIT_SEQUENCE_EDITOR environment variable and/or the sequence.editor
configuration variable can be used to specify such an editor, while
allowing the usual editor to be used to edit commit log messages. As
usual, the environment variable takes precedence over the configuration
variable.

It is envisioned that other "sequencer" based tools will use the same
mechanism.

Signed-off-by: Peter Oberndorfer <kumbayo84@arcor.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>