Now we can stage lines by range and we can also stage reversals
meaning we can delete lines or move lines from the working tree
to the index and vice versa.
I looked at how a few different git guis achieved this to iron out
some edge cases, notably ungit and git cola. The end result is
disstinct from both those repos, but I know people care about
licensing and stuff so I'm happy to revisit this if somebody
considers it derivative.
This fixes up some git and oscommand tests, and pulls some tests into commit_list_builder_test.go
I've also made the NewDummyBlah functions public so that I didn't need to duplicate them across packages
I've also given OSCommand a SetCommand() method for setting the command on the struct
I've also created a file utils.go in the test package for creating convient 'CommandSwapper's, which
basically enable you to assert a sequence of commands on the command line, and swap each one out for
a different one to actually be executed
