Implement "git fetch --prune --unshallow"

[litetex]

Implement --unshallow when lib2git finally implements it (after 8 years lol) -> libgit2/libgit2sharp#229
Currently "git fetch --prune --unshallow" has to be run before this task...

If this is not going to happen soon:

• Run without /git/workspace and clone directly...

[litetex]

Exception without unshallow:

LibGit2Sharp.NotFoundException: object not found - no match for id (<id>)

[litetex]

libgit2#5254