Add support for custom extensions

[captn3m0]

Refs:

• https://github.com/libgit2/libgit2/pull/6031
• https://github.com/libgit2/libgit2sharp/pull/1908
• https://github.com/rust-lang/git2-rs/pull/791

[captn3m0]

I haven't traced this down yet, but this is becoming a showstopper with libgit upgrade to 1.6.

Setup

Create a bare repository with the partialClone extension:

mkdir r
git init --bare .
git config  extensions.partialClone true
git fetch --quiet --tags --filter=blob:none --depth=1 https://github.com/captn3m0/jekyll-sqlite.git

Test script:

require 'rugged'
repo = Rugged::Repository.bare 'r'

It fails on rugged 1.6.2+:

test.rb:3:in `bare': unsupported extension name extensions.partialclone (Rugged::RepositoryError)
	from test.rb:3:in `<main>'

but works fine on rugged 1.5.1.