Fetch Git repos with libgit2 instead of Git command line

[christianspecht]

Make a second implementation of the GitScm class, using LibGit2Sharp (instead of command line Git) under the hood.

The current command line implementation should be the default, the new implementation should be enabled via config setting.

If libgit2sharp can do everything we need, maybe it's a solution for #57 and #58.

[christianspecht]

• https://github.com/libgit2/libgit2sharp/wiki/git-init
• https://github.com/libgit2/libgit2sharp/wiki/git-fetch