Replace nodegit with isomorphic-git
nodegit has proven to be too difficult to support. It has very limited portability (especially on Linux) and the workaround of recompiling imposes too many prerequisites and takes too long. A pure JavaScript git client named isomorphic-git has emerged since development of Antora started and is now a viable alternative. It may also solve some other shortcomings we've encountered in nodegit, such as a poor authentication mechanism, lack of support for symlinks, and no integration with git-lfs. For these reasons and more, it's time to switch Antora to isomorphic-git.
While making this change, we could still consider the architecture proposed in #93 (closed).