This change causes `zmanage update` to track the submodules as described
in origin/master, not from their upstream (dirty) states. Users wishing
to "revert" this change on occasion to test some upstream submodule
features, do: `cd ~/.zim && gSu`, assuming you have the git aliases.
This will pull the upstream modules to their latest state, leaving a
dirty HEAD state with these untracked modules. `cd ~/.zim && gSI` to
revert.
Closes#127
