git-commit-prev-message: fix first usage

Variable `log-edit-comment-ring-index' has to be buffer-local but isn't
marked as automatically buffer-local and since `git-commit-mode' cannot
derive from `log-edit-mode' we too have to explicitly mark it as such
in `git-commit-setup'.

`git-commit-prev-message' differs from `log-edit-previous-comment' in
that it saves the current buffer content in the message ring before
replacing it in the buffer with the previous message using the latter.

`log-edit-comment-ring-index' is nil until `log-edit-previous-comment'
is first used, at which point it is initialized to point to the last
(if going backward) or the last (if going forward) message in the ring.
Because the last message is the one we just save, we have initialize
ourselves *before* calling `log-edit-previous-comment' otherwise we
would "go back" to the current message.  When going forward or when
there was no message to be save this is irrelevant, or rather doing
so would actually be counterproductive, so we don't.