Move away from git CLI dependency to libgit2

Currently, stdlint assumes that `git` command is available on the shell `stdlint` is run. However, this is not the case mostly especially in situations where stdlint is run inside a stripped-down container. See if we can instead write a wrapper to libgit2 (https://libgit2.org/), or there is an existing wrapper we can use. (see: https://www.npmjs.com/search?q=libgit2)