Auto merge of #1915 - alexcrichton:downgrade-libgit2, r=huonw

Unfortunately this flew past on the bots because we didn't built on one of the
10.7 bots, but it looks like libgit2 doesn't currently build on OSX 10.7 so
we'll have to wait until that's fixed to update.

The tracking issue for fixing this in libgit2 is: https://github.com/libgit2/libgit2/issues/3386