[tools] zmanage update: track submodules

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

tools/zim_update

@@ -4,8 +4,8 @@
 
 cd ${ZDOTDIR:-${HOME}/.zim}
 
-# this is the cleanest way I know of to update a repository
+# this is the cleanest way I know how to update a repository
 git remote update -p
 git merge --ff-only @\{u\}
 # and update the submodules
-git submodule foreach --quiet --recursive git pull origin master
+git submodule update --init --recursive