rebase -i: redo tasks that die during cherry-pick

rebase -i: redo tasks that die during cherry-pick

When rebase--interactive processes a task, it removes the item from
the todo list and appends it to another list of executed tasks. If a
pick (this includes squash and fixup) fails before the index has
recorded the changes, take the corresponding item and put it on the todo
list again. Otherwise, the changes introduced by the scheduled commit
would be lost.

That kind of decision is possible since the cherry-pick command
signals why it failed to apply the changes of the given commit. Either
the changes are recorded in the index using a conflict (return value 1)
and rebase does not continue until they are resolved or the changes
are not recorded in the index (return value neither 0 nor 1) and
rebase has to try again with the same task.

Add a test cases for regression testing to the "rebase-interactive"
test suite.

Signed-off-by: Fabian Ruch <bafain@gmail.com>
Signed-off-by: Phil Hord <hordp@cisco.com>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>