sequencer: fix edit handling for cherry-pick and revert messages

sequencer: fix edit handling for cherry-pick and revert messages

save_opts() should save any non-default values.  It was intended to do
this, but since most options in struct replay_opts default to 0, it only
saved non-zero values.  Unfortunately, this does not always work for
options.edit.  Roughly speaking, options.edit had a default value of 0
for cherry-pick but a default value of 1 for revert.  Make save_opts()
record a value whenever it differs from the default.

options.edit was also overly simplistic; we had more than two cases.
The behavior that previously existed was as follows:

                       Non-conflict commits    Right after Conflict
    revert             Edit iff isatty(0)      Edit (ignore isatty(0))
    cherry-pick        No edit                 See above
    Specify --edit     Edit (ignore isatty(0)) See above
    Specify --no-edit  (*)                     See above

    (*) Before stopping for conflicts, No edit is the behavior.  After
        stopping for conflicts, the --no-edit flag is not saved so see
        the first two rows.

However, the expected behavior is:

                       Non-conflict commits    Right after Conflict
    revert             Edit iff isatty(0)      Edit iff isatty(0)
    cherry-pick        No edit                 Edit iff isatty(0)
    Specify --edit     Edit (ignore isatty(0)) Edit (ignore isatty(0))
    Specify --no-edit  No edit                 No edit

In order to get the expected behavior, we need to change options.edit
to a tri-state: unspecified, false, or true.  When specified, we follow
what it says.  When unspecified, we need to check whether the current
commit being created is resolving a conflict as well as consulting
options.action and isatty(0).  While at it, add a should_edit() utility
function that compresses options.edit down to a boolean based on the
additional information for the non-conflict case.

continue_single_pick() is the function responsible for resuming after
conflict cases, regardless of whether there is one commit being picked
or many.  Make this function stop assuming edit behavior in all cases,
so that it can correctly handle !isatty(0) and specific requests to not
edit the commit message.

Reported-by: Renato Botelho <garga@freebsd.org>
Signed-off-by: Elijah Newren <newren@gmail.com>
Reviewed-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>