rebase -i: fix adding failed command to the todo list

rebase -i: fix adding failed command to the todo list

When rebasing commands are moved from the todo list in "git-rebase-todo"
to the "done" file (which is used by "git status" to show the recently
executed commands) just before they are executed. This means that if a
command fails because it would overwrite an untracked file it has to be
added back into the todo list before the rebase stops for the user to
fix the problem.

Unfortunately when a failed command is added back into the todo list the
command preceding it is erroneously appended to the "done" file.  This
means that when rebase stops after "pick B" fails the "done" file
contains

	pick A
	pick B
	pick A

instead of

	pick A
	pick B

This happens because save_todo() updates the "done" file with the
previous command whenever "git-rebase-todo" is updated. When we add the
failed pick back into "git-rebase-todo" we do not want to update
"done". Fix this by adding a "reschedule" parameter to save_todo() which
prevents the "done" file from being updated when adding a failed command
back into the "git-rebase-todo" file. A couple of the existing tests are
modified to improve their coverage as none of them trigger this bug or
check the "done" file.

Reported-by: Stefan Haller <lists@haller-berlin.de>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>