topic ruby18: Add support for Ruby 1.8\n
Adds support for Ruby 1.8

Sadly, IO.popen does not support multiple arguments on ruby 1.8. That means we have to build the entire command by merging all parts together; I hope escaped_command is good enough for this :(
Merge from topic branch: 'ruby18'
topic finish_topic: Adds a git topic finish command.\n
Adds the git-topic finish subcommand
Merge from topic branch: 'finish_topic'
Updates documentation
Merge from topic branch: 'finish_topic'