Pull doesn't work

``` ruby
g = Git.open "repo"
g.pull
# "Already up-to-date."
g.lib.send(:command, 'pull')
# "Updating 6d57a5a..0bbe197\nFast-forward\n file.rb |    6 +-----\n 1 files changed, 1 insertions(+), 5 deletions(-)"
```

It looks like internally Git::Base#pull doesn't use git pull at all, but fetches and merges. Why does it not use git pull as expected? Currently it just reports it's already up to date.
