doc: mention GIT_SEQUENCE_EDITOR and 'sequence.editor' more

doc: mention GIT_SEQUENCE_EDITOR and 'sequence.editor' more

The environment variable `GIT_SEQUENCE_EDITOR`, and the configuration
variable 'sequence.editor', which were added in 821881d88d ("rebase -i":
support special-purpose editor to edit insn sheet, 2011-10-17), are
mentioned in the `git config` man page but not anywhere else.

Include `config/sequencer.txt` in `git-rebase.txt`, so that both the
environment variable and the configuration setting are mentioned there.

Also, add `GIT_SEQUENCE_EDITOR` to the list of environment variables
in `git(1)`.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>