git: show the version of `pygit2` with verbose version output

Authored by mharbison72.


git: show the version of pygit2 with verbose version output

This seems like useful info to have when debugging. I followed the precedent of
hg-git, which prints something like:

hggit        external  0.9.0a1 (dulwich 0.19.15)

We don't have a version number assigned (because it's internal), so it's just
the parenthetical.

Differential Revision: https://phab.mercurial-scm.org/D9436