If the attacker has access to your remote repo then isnt that the problem? You can sign your commits all day but you have more serious issues to attend to.
At least if things go horribly wrong, by having all your commits signed you can instantly point out where the illicit tampering happened and have some proof that the illicit change wasn't willingly added by you
I started to sign all my commits in bzr, hg and git since more than 1 year, and it's very easy to do it: you should try to do it as well
(I disagree with Linus and other people who argue that you should just sign the merge commits, since otherwise you'd be "inflationing"/reducing the value of signatures if you'd sign everything: it would be like only signing the very important messages with gpg, and not doing it for the remaining majority: it's fine if you want to have deniability for your unsigned messages, but that's not what I want for my commits: ideally I'd want all of the changes to be auditable)
