Improve integration specs.

* Add missing _spec prefix to one file
* Rename ambiguous specs
* Merge duplicate contexts
* Fix typos
Support commenting of partial lines.

e.g. `add . # Add all files`
Add a :help command

`:help` on its own will show a list of internal commands, and `:help
command` will show help for the given internal command.

This commit also extracts the various internal command specs from
spec/unites/commands/internal_command_spec.rb into their own files.
Add :help to the man page.