Generate missing libgit2 coverage report

[josharian]

I keep discovering missing bits of libgit2 in git2go. Along the lines of #427, it strikes me that it'd be useful to have a coverage report (maybe run in CI?) that lists missing pieces. A simple implementation would be to parse the headers to find all exported libgit2 API. Then just grep (strings.Contains) for those strings in the git2go implementation. Not foolproof, but it'd go a pretty long ways.

Thoughts?

(Also, seems like the maintainers of git2go are perhaps on vacation? I'll keep sending PRs for now, but at some point I'm going to give up pending some indication that someone will eventually review.)