Print future commands on failure.

`Git::run_safe` will print all commands that would have ran after the
specific command that had failed to complete.
Make `hotfix merge` use only minimal `Git::run_safe`.

Add param to `submodules_update` that allows one to return a string of
the command rather than execute it.
Assign command to variable for readability's sake.
Merge print-commands-on-failure (#67) into master

Print future commands on failure.

`Git::run_safe` will print all commands that would have ran after the
specific command that had failed to complete.