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.
I'm not too happy about fragment generation in CommitDiffPage.java and the id generation here. In CommitDiffPage, you just use `'#' + entry.path` for the href of the link. So what does Wicket do with this if the path contains critical characters like ", ', or a blank? In HTML 5, ids must not contain blanks, and in XHTML, they're even more constrained (and in particular slashes are not allowed). Perhaps one should better use `'n' + DiffEntry.getNewId().name()` (or `getOldId()` for deletions) in both places as fragments and ids; that would be guaranteed not to contain any problematic characters and to be a valid fragment or id according to any HTML specification.