Faster try builds!

When we run a try build, we wipe out the build directory between each job; we want to make sure that every user's build has a fresh environment to build in.

Unfortunately this means that we also wipe out the clone of the try repo, and so we have to re-clone try every time.

On Linux and OSX we were spending an average of 30 minutes to re-clone try, and on Windows 40 minutes. The majority of that is simply 'hg clone' time, but a good portion is due to locks: we need to limit how many simultaneous build slaves are cloning from try at once, otherwise the hg server blows up.

Way back in September, Steve Fink suggested using hg's share extension to make cloning faster.

Then in November, Ben Hearsum landed some changes that paved the way to actually turning this on.

Today we've enabled the share extension for Linux (both 32 and 64-bit) and OSX 10.6 builds on try. Windows and OSX 10.5 are coming too, we need to upgrade hg on the build machines first.

Average times for the 'clone' step are down to less than 5 minutes now.

This means you get your builds 25 minutes faster! It also means we're not hammering the try repo so badly, and so hopefully won't have to reset it for a long long time.

We're planning on rolling this out across the board, so nightly builds get faster, release builds get faster, clobber builds get faster, etc...