use hg export instead of hg show

[davide125]

In https://github.com/facebook/between-meals/blob/master/lib/between_meals/repo/hg.rb#L153 you should use 'hg export' instead of 'hg show' for better compatibility with standard mercurial installations (where 'hg show' dumps the current mercurial config instead of outputting the last commit in the repo). This may require adapting the regexps as well as the format is slightly different.