ring-index must be non-nil if we saved a message

When ring-index is nil, log-edit-previous-comment goes to the "last"
item, which turns out to be the current message that has just been
saved, i.e. the message in buffer does not change.
Merge pull request #85 from npostavs/no-nil-index

ring-index must be non-nil if we saved a message
Makefile: actually use BATCHC