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?

首先阅读 build.libgit2sharp.sh，并跟踪它如何构建 libgit2、生成二进制文件，以及在 Linux 和 macOS 上运行测试。与维护者确认预期结果是编写脚本文档，还是拆分出独立的构建和测试脚本；当工作流得到清晰说明或被整洁地拆分时，即视为完成。