✨ new subcommand `pull` to update nested modules

[jeftadlvw]

Maybe you want to pull every submodule at once, instead of cd-ing into every directory and performing the pull manually. This new subcommand should provide this functionaliy.

Some notes:

• don't perform a sync beforehand, as some modules are checked out at different branches/commits manually by choise
• git pull will fail on detached repository HEAD's
• git pull will output an error if local unsafed/untracked changes exist and would be overwritte by a merge
• in case of merge conflicts, abort the merge using git merge --abort

Performing a multi-module pull should be non-destructive.

[jeftadlvw]

The subcommand and tests are implemented. This feauture request is done. Please open a new issue if you experience any problems with this feature! Thank you!