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 821881d ("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>

Documentation/git-rebase.txt

@@ -204,6 +204,7 @@ CONFIGURATION
 -------------
 
 include::config/rebase.txt[]
+include::config/sequencer.txt[]
 
 OPTIONS
 -------

Documentation/git.txt

@@ -605,6 +605,12 @@ other
 	an editor is to be launched. See also linkgit:git-var[1]
 	and the `core.editor` option in linkgit:git-config[1].
 
+`GIT_SEQUENCE_EDITOR`::
+	This environment variable overrides the configured Git editor
+	when editing the todo list of an interactive rebase. See also
+	linkit::git-rebase[1] and the `sequence.editor` option in
+	linkit::git-config[1].
+
 `GIT_SSH`::
 `GIT_SSH_COMMAND`::
 	If either of these environment variables is set then 'git fetch'