Fixed The issue with sumatra repeat and diff (only git)

[maharjun]

This is in reference to issue #354. I have implemented a fix as follows

1. Added the functions reset and patch to the WorkingCopy Interface and implemented them for git, unfortunately I do not have any experience with the other API's and so I cannot implement them
2. Updated update_code to take a diff string as an argument and apply diffs
3. Updated the Record.launch function to take diff as a keyword argument so that diff stored code can also be launched.
4. Updated the Project.Repeat function to reset the diff along with commit version at the end of run.

[apdavison]

This could do with some tests, but I will implement them as part of extending this to the other version control systems (#365)