Pushed to release-1.7.x (5c55e7a)

It turns out that most of the original patch was correct. We DO want to be using id=. The reasoning was wrong and the patch was incomplete.

Reasoning: id2= is actually only used in diffs (and has been this way since 2007). If id= is not specified, it defaults to 'master' which was why most reviews were working anyway (since most development happens on the master branch of most projects). id2= was actually completely ignored and had no effect at all. (The red herring is that the sources occasionally refer to the value of id2= as "sha2", but that's supposed to be read as "the second SHA hash" and not as "SHA-2".

Incomplete: The Fedora Hosted hosting service was creating repos with a raw file URL referencing /plain/ rather than /blob/. /plain/ works much of the time, but not if the file is not pure ASCII. This was the cause of the behavior that forced me to request a reversion of the original patch, but it turns out things would have been fine for regular cgit repositories (since the documentation properly listed /blob/). I also forgot to update the tests to match the change.