Tweak how <tt> blocks in manpages are rendered

See issue #243. This is an attempt to make the manpages render somewhat nicer. It's not great, but with this minimal change, I find that my reading experience is already greatly improved compared to the current state. Here is an example taken from the git-fast-import manpage. Before:

![Screen Shot 2013-01-30 at 15 18 38](https://f.cloud.github.com/assets/241512/111096/10b00dc8-6ae8-11e2-80cd-d497305e1fab.png)

After:

![Screen Shot 2013-01-30 at 15 19 10](https://f.cloud.github.com/assets/241512/111097/16af07ce-6ae8-11e2-83af-0f112f0f7be7.png)


Perhaps the red should be changed, though, to perhaps something a bit darker, or even the same color as the rest of the text. I also experimented with just changing this to use typewriter, but no other effects. But I found the result a bit too bland:

![Screen Shot 2013-01-30 at 15 21 19](https://f.cloud.github.com/assets/241512/111103/6c525b72-6ae8-11e2-9b9b-ff62fa0bc926.png)

However, tastes differ. If you want this to go into a somewhat different direction, please give me some hints as to how, and I can make some more variants and screenshots thereof. Of course to really see the full effect, one should consider much more of the reference pages than just that tiny snippet I am showing here.