sequencer (rebase -i): implement the 'edit' command

sequencer (rebase -i): implement the 'edit' command

This patch is a straight-forward reimplementation of the `edit`
operation of the interactive rebase command.

Well, not *quite* straight-forward: when stopping, the `edit`
command wants to write the `patch` file (which is not only the
patch, but includes the commit message and author information). To
that end, this patch requires the earlier work that taught the
log-tree machinery to respect the `file` setting of
rev_info->diffopt to write to a file stream different than stdout.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>