fails when run in a submodule folder

[depressiveRobot]

When running git up in a submodule folder, the following error occurs:

Fetching origin
/usr/local/lib/ruby/gems/2.2.0/gems/grit-2.5.0/lib/grit/git.rb:120:in `read': Not a directory @ rb_sysopen - /Users/*****/SUBMODULE/.git/HEAD (Errno::ENOTDIR)
    from /usr/local/lib/ruby/gems/2.2.0/gems/grit-2.5.0/lib/grit/git.rb:120:in `fs_read'
    from /usr/local/lib/ruby/gems/2.2.0/gems/grit-2.5.0/lib/grit/ref.rb:64:in `current'
    from /usr/local/lib/ruby/gems/2.2.0/gems/grit-2.5.0/lib/grit/repo.rb:230:in `head'
    from /usr/local/lib/ruby/gems/2.2.0/gems/git-up-0.5.12/lib/git-up.rb:192:in `returning_to_current_branch'
    from /usr/local/lib/ruby/gems/2.2.0/gems/git-up-0.5.12/lib/git-up.rb:21:in `block (2 levels) in run'
    from /usr/local/lib/ruby/gems/2.2.0/gems/git-up-0.5.12/lib/git-up.rb:183:in `with_stash'
    from /usr/local/lib/ruby/gems/2.2.0/gems/git-up-0.5.12/lib/git-up.rb:20:in `block in run'
    from /usr/local/lib/ruby/gems/2.2.0/gems/grit-2.5.0/lib/grit/git.rb:90:in `with_timeout'
    from /usr/local/lib/ruby/gems/2.2.0/gems/git-up-0.5.12/lib/git-up.rb:19:in `run'
    from /usr/local/lib/ruby/gems/2.2.0/gems/git-up-0.5.12/bin/git-up:5:in `<top (required)>'
    from /usr/local/bin/git-up:23:in `load'
    from /usr/local/bin/git-up:23:in `<main>'

[grepsedawk]

Was just about to make this issue.

[d4rkd0s]

Related to #67