Reported by a user on email, since a few versions ago (probably since the upgrade of the Xcode SDK to 10.8+) the commit text view has had Smart Quotes and Smart Dashes enable by default (right-click, Substitution menu). This cases a double-dash and quotes to be converted to extended characters which can cause problems on other platforms.
While the user can disable these options, they are not remembered.
This is exactly the same as the spell checking option in the commit text dialog, which is currently remembered. Rather than adding more options here, the best thing to do is to disable these options by default instead, since for a commit message you don't really want automatic substitution with special characters.