Supporting diffs in Github links

[VirdanTheBurden]

The current system that listens for GitHub links to format and post in a channel cannot handle URLs that point to a diff.

Supporting these types of links as well would allow for users to specifically demonstrate changes that occur in a series of commits, and also reduce future confusion on what kinds of links elicit a response from the bot.

[wookie184]

Is the idea to display entire diffs, or only selected lines in the diff? If it's the entire diff, I think it'd often be too long to fit in a message. Only displaying selected lines could work, though I've found GitHub's UI for selecting/viewing specific lines in diffs isn't as good as for normal files, and (the reason I mention this) this seems potentially quite difficult to implement, given how the GitHub UI lets you select using line numbers from both things being compared.

I think it would help to see a specific case where this could be helpful, to get a better idea of how it would work.