Highlight Git commit summary with orange color

Fix chriskempson/tomorrow-theme#181.

Before:
![screenshot 2014-08-23 22 18 04](https://cloud.githubusercontent.com/assets/853977/4020679/2054975c-2ac8-11e4-9fbd-b04c8e7f820a.png)

After:
![screenshot 2014-08-23 22 18 31](https://cloud.githubusercontent.com/assets/853977/4020680/2337460e-2ac8-11e4-8752-d838d413fe77.png)