main: highlight long commit titles

Many developers and tools respect the Git convention for commit title,
as discussed in the git-commit(1) man page:

> Though not required, it's a good idea to begin the commit message with
> a single short (less than 50 character) line summarizing the change

This patch adds the ability to highlight text exceeding 50 characters in
commit titles, through a "show-long-title" toggle.

Here's what it looks like:
![Highlight long commit title in tig](http://i.imgur.com/dNRK5Fw.png)

Cheers,
Vivien