update: add support for an argument

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
update: silence local fetches

The message is just annoying and there's no need for that.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
update: pass ff-only option when in ff-only mode

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
update: improve sed expression

So that delimiter markers in the message can't be an issue.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
update: remove squash option

Doesn't make much sense in `git update` context.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>