Improve the commitdiff.

Same as PR-225, but against gitblit:develop.

Width of the line number columns reduced to 2px padding and 3em width. On my screen the three columns (two line numbers and +/-) are 95px, vs. the two line number columns at dev.gitblit at 82px.

Sorry for the duplicate PR, but GitHub didn't let me rewrite the original PR (which was based on master). I just hope I didn't mess up somewhere in this rebase confusion.
Doesn't `line` need to be run through `StringUtils.escapeForHtml()` here?