Update rsync.rb

Problem on OS X that seems to be related to cwd when doing git reset --hard master/origin. By wrapping the commands in a Dir.chdir block the git error message goes away.

Error message I keep getting is of the following form:

git --git-dir tmp/deploy/.git --work-tree tmp/deploy reset --hard origin/master
error: packfile tmp/deploy/.git/objects/pack/pack-ac917f6f3e4ecd079fc654cb5acd6586840f2f97.pack cannot be accessed
HEAD is now at ae0366c testing mina-rsync