Implement mix-messy-docs L2 exercise

[woojiahao]

Exercise Review

Exercise Discussion

#70

Checklist

• If you require a new remote repository on the Git-Mastery organization, have you created a request for it?
• Have you written unit tests using repo-smith to validate the exercise grading scheme?
• Have you tested the download script using test-download.sh?
• Have you verified that this exercise does not already exist or is not currently in review?
• Did you introduce a new grading mechanism that should belong to git-autograder?
• Did you introduce a new dependency that should belong to app?

[woojiahao]

@damithc my experience implementing this exercise: implementing an L2 isn't very different from an L1 exercise, except that we might need to be slightly cleverer in setting up the scenario (implementing remote branch tracking) and testing (simulating a merge due to a merge conflict)

[woojiahao]

I'm still fleshing out the unit tests, but we should be able to get the L2s out by the weekend for students to attempt!