Update package archives
Update package yasnippet-20140729.1240
Fix borked call-process in update-packages.el

Also dump results of git calls to a buffer, in case there prove to be
more issues.
Update package flycheck-20140808.424
Simplify update-packages' git-add command

Better to run once than twice, and also to be compatible with git 2.0,
whenever that ships for OS X.