git,zsh:autoload: Move spull into own function

Move `git-track` into its own autoloadable function.

This way it is better maintainable as in one-liner format and brings the
possibility of having more complexity (e.g. for better portability).

By that it is also now executable as `git-spull` as well as `git spull`
as before.

.config/zsh/autoload/git/git-spull

@@ -0,0 +1,7 @@
+#!/usr/bin/env zsh
+
+emulate -L zsh -o err_return
+
+git pull "$@"
+git submodule sync --recursive
+git submodule update --init --recursive