resolves #199: command list architecture

@kevgo 

At the moment only git-sync has continue scripts. After this is merged I will add continue scripts to each command that could support it one at time.

* I created new stash_open_changes and restore_open_changes functions in order to get rid of the need to set variables in abort / command list / undo scripts.
* Removed apart sync_branch as a command in that may error and its easier to see without it. 
* Removed pull_branch as it was causing a fetch to occur in the continue scripts
* Created a temp_filename function as the bash linter didn't like having a file being read and outputted to in the same line
Can you make it so that `pop_command` returns the command that it popped, similar to how `peek_command` returns the command that it peeks? This is a bit more conforming to existing naming conventions for stacks.