Extract method subcommand_execute in Pull

All subcommands of pull start their execution with the same logic.
Fix bug in Formatter#change_viz

Check the comment attached.
Threw an error when a pull request without line changes was submitted.
First running implementation of the merge button
Fix mergeability string in Formatting

GitHub says 'able to merge', let's use that too.
Add info for dirty pull requests in Pull::Show
Refactor pr in Pull

The pull request (pr) can now be accessed through a method.
Add methods to compare head and base in Pull

Useful to know if a rebase is needed.
Show if a rebase is needed in Pull::Show

Refactors the formatting methods a bit as well.
Handle errors and warning in Pull::Merge
Shorten option description for ghi pull merge -m
Allow to pull or rebase locally after merge

Does nothing if both (-p and -r) are requested, otherwise goes to the
base branch of the pull request and pulls in the merged PR - either
with pull or pull --rebase.