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?

Comece lendo build.libgit2sharp.sh e acompanhando como ele compila o libgit2, gera os binários e executa os testes no Linux e no macOS. Confirme com os maintainers se o resultado pretendido é a documentação do script ou scripts separados de build e teste; considera-se concluído quando o fluxo de trabalho estiver claramente explicado ou devidamente separado.