[WIP] Commit and Push commands

[LinuxMercedes]

This pair of commands will allow people to push local changes to students' repos. For example, autograder output can be added and pushed once grading is complete.

We have no plans to add a "copy this file to all local student repos" feature. If necessary, this can be done from a shell like so:

for dir in hw1/*; do [[ -d $dir ]] && cp updated-file $dir/original-file; done

The push command is written to, by default, require you to lock student repos before pushing to them. This will reduce the possibility of conflicts; however, it is impossible to avoid conflicts on the students' side if they have un-pushed commits before assigner push is run. We recommend using this feature very carefully if students are intended to continue working in their repos after the push occurs (i.e., if assigner push is used to update an assignment after it has been assigned and opened).

Closes #98, closes #138.

TODO:

• Changelog
• Tutorial docs
• Integration tests

[LinuxMercedes]

Added a neat commit tip to the tutorial; fixes #138!