After this exchange on stack overflow(1) I thought I would point out that Textmate seems incapable of faking italics on fonts without an italic face, such as Source Code Pro(2).
It’s easily solved by switching fonts.
Do with this information as you like.
 http://stackoverflow.com/questions/26786066/how-to-set-font-style-in-a-textm...  https://github.com/adobe-fonts/source-code-pro
Keep up the good work!
Graham P Heath