Add API to get diff for statusline

[Shatur]

Describe the solution you'd like

In the statusline, users often display numbers with the number of additions / changes / deletions. Since already vgit processes this information to place signs, it would be convenient to be able to get it using lua.
For example, with gitsigns plugin I can get this information using vim.b.gitsigns_status_dict.

[ibhagwan]

@Shatur, take a look at #218, if you wish to try it out use my fork for now.