Short GitHub URLS

[ipmb]

via @hamiltont at BotBotMe/botbot-bot#30

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I don't see an issue for this, so here's one in case it's just never been brought up.

Currently this is the URL format:

$ gh#1129
$ Objectify FrameworkTest Types: https://github.com/TechEmpower/FrameworkBenchmarks/pull/1129

I'd lvoe to see this instead

$ gh#1129
$ Objectify FrameworkTest Types: http://git.io/7P6RkA

This is what Travis-CI does for their IRC notifications and IMO it's much cleaner. It's also a relatively trusted URL - I think git.io will only shorten URLs to github

[ipmb]

If anybody wants to look at this, this is the module: https://github.com/BotBotMe/botbot-plugins/blob/master/botbot_plugins/plugins/github.py