sequencer: avoid adding exec commands for non-commit creating commands

sequencer: avoid adding exec commands for non-commit creating commands

The `--exec <cmd>` is documented as

    Append "exec <cmd>" after each line creating a commit in the final
    history.
    ...
    If --autosquash is used, "exec" lines will not be appended for the
    intermediate commits, and will only appear at the end of each
    squash/fixup series.

Unfortunately, it would also add exec commands after non-pick
operations, such as 'no-op', which could be seen for example with
    git rebase -i --exec true HEAD

todo_list_add_exec_commands() intent was to insert exec commands after
each logical pick, while trying to consider a chains of fixup and squash
commits to be part of the pick before it.  So it would keep an 'insert'
boolean tracking if it had seen a pick or merge, but not write the exec
command until it saw the next non-fixup/squash command.  Since that
would make it miss the final exec command, it had some code that would
check whether it still needed to insert one at the end, but instead of a
simple

    if (insert)

it had a

    if (insert || <condition that is always true>)

That's buggy; as per the docs, we should only add exec commands for
lines that create commits, i.e. only if insert is true.  Fix the
conditional.

There was one testcase in the testsuite that we tweak for this change;
it was introduced in 54fd3243da ("rebase -i: reread the todo list if
`exec` touched it", 2017-04-26), and was merely testing that after an
exec had fired that the todo list would be re-read.  The test at the
time would have worked given any revision at all, though it would only
work with 'HEAD' as a side-effect of this bug.  Since we're fixing this
bug, choose something other than 'HEAD' for that test.

Finally, add a testcase that verifies when we have no commits to pick,
that we get no exec lines in the generated todo list.

Reported-by: Nikita Bobko <nikitabobko@gmail.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Acked-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>