Make the Linux/MacOSX experience slightly more delightful

#786 brings under the light the fact that there's still a huge room for improvment regarding the following topics

• How to build libgit2
• How to run the tests against the generated binaries

Currently, all of the above is being automagically done by the build.libgit2sharp.sh script.

I can think of two options:

• Add some documentation in the script header describing what the script does, where the binaries are generated
• Break up the script into two scripts (ie. build and runtests)

There are certainly others ways to solve this.

Thoughts?

まず build.libgit2sharp.sh を読み、libgit2 のビルド、バイナリの生成、Linux および macOS でのテスト実行の流れを追ってください。意図された成果がスクリプトのドキュメント化なのか、ビルド用とテスト用の別々のスクリプトなのかを maintainer に確認してください。ワークフローが明確に説明されるか、きれいに分離されれば完了です。