Allow to create patches w/o a/ and b/ prefixes

Hi,

Creating SVN compatible patches is useful in a lot of cases and espacially when you are using Git over SVN or when you share diffs with others before to commit. Of course that this can be done from the command line with "git diff --no-prefix master..branch > patch.diff", but it's quite annoying when you have more that 2-3 commits to share.

In the TurtoiseGIT, there is already the FormatPatch window that creates these kind of diffs but in the Git format. I think keeping the same window but having another checkbox in the window, would easily fix the issue.

Having another checkbox near 'Send Mail after create' would be an option and maybe increasing the paddings near these two checkboxes to be aligned with the padding from the 'Output Directory' and 'Version' groups.

Thanks !