Don't include the possible leading sign in the span of digits to be grouped as this gave nonsensical strings such as "-,123" when adding thousands separators to "123". Closes #14526. git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72256 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775