Use libgit2

[Freed-Wu]

libgit2 have official binding for nodejs. Perhaps we can use it to replace all process calling for git to save process fork cost.

[chemzqm]

Failed to install by npm install gitteh

npm error path /module/node_modules/gitteh
npm error command failed
npm error command sh -c node install.js
npm error fs.js:45
npm error } = primordials;
npm error     ^
npm error
npm error ReferenceError: primordials is not defined
npm error     at fs.js:45:5

[chemzqm]

It requires CMake 2.6 and gcc to install, seems not good idea for coc-git