wrap git fetch/reset in Dir.chdir(rsync_stage)

Environment: OS X 10.7.5, bash 4.3.11(1), ruby 2.1.1p76, gem 2.2.2

    File: lib/mina/rsync.rb 
    In task stage
        git reset --hard origin/#{settings.branch} was failing with the following:

    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

This seems to be a bug in Git but wrapping git fetch/reset in Dir.chdir("#{settings.stage}") fixed the issue