var: add GIT_SEQUENCE_EDITOR variable

var: add GIT_SEQUENCE_EDITOR variable

The editor program used by Git when editing the sequencer "todo" file
is determined by examining a few environment variables and also
affected by configuration variables. Introduce "git var
GIT_SEQUENCE_EDITOR" to give users access to the final result of the
logic without having to know the exact details.

This is very similar in spirit to 44fcb497 (Teach git var about
GIT_EDITOR, 2009-11-11) that introduced "git var GIT_EDITOR".

Signed-off-by: Sean Allred <allred.sean@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>