Feature request #1879: Turn SHA-1s in commit messages into hyperlinks in repo browser

I can't do this server side because at that time the text goes through the filter, the filter doesn't know what page it is being run on nor the request.

So, will have to do it client-side via JS. Already doing some magic anyway for syntax highlighting; doing a little more of this kind of thing won't be too bad.

Will start by targeting simple SHA-1s where the risk of false positives is low. Can also think about looking at some other special cases such as "the foo branch" linking to, um, the foo branch and so on.