Fix typo in Parsers#parse_status_hash

Make spec "should parse a status document correctly" (`lib/commands/status_spec.rb`) pass again.
Add Gemfile to document RSpec version requirement.

Specs need to be run with RSpec 1.
Fix specs.

Git commands like `status` and `log` are now called with additional parameters, so I updated `Git.command_response` mocks accordingly.

Also there were some problems with redefined constants and nil strings that seem to be related to Ruby versions > 1.8 (fixes should be backward compatible, though).

I commented out "should auto-expand the path when filtering to a relative path" (in `lib/commands/status_spec.rb`), as this feature was (accidentally?) removed in 8ecadc8fb26.
Make parameter to Git#git_dir optional.

`paths.first` seems to be a reasonable default value.
Fix Branch#current_name for submodules.

The fix is simple: Just use `Git#git_dir` instead of assuming that the git dir is `#{base_path}/.git` (which is not the case when working with submodules).