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?

先閱讀 build.libgit2sharp.sh，追蹤它如何建置 libgit2、產生二進位檔，以及在 Linux 和 macOS 上執行測試。與維護者確認預期結果是撰寫指令碼文件，還是拆分出獨立的建置與測試指令碼；當工作流程獲得清楚說明或被妥善分離時，即表示完成。