Add GIT_EDITOR environment and core.editor configuration variables

Add GIT_EDITOR environment and core.editor configuration variables

These variables let you specify an editor that will be launched in
preference to the EDITOR and VISUAL environment variables. The order
of preference is GIT_EDITOR, core.editor, EDITOR, VISUAL.

[jc: added a test and config variable documentation]

Signed-off-by: Adam Roben <aroben@apple.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>