Re: [RFC PATCH v1 2/4] libgit-sys: add symlink to git repo root and clean after build

> Unlike `cargo build`, `cargo package` does not get access to the entire Git repo
> containing a Rust crate. Instead, it prepares a directory starting from the
> crate root (potentially excluding files, such as those not under version
> control, or explicity excluded in the Cargo.toml file).
> 
> This means that the current method of building the libgit-sys crate does not
> work with `cargo package`, as it tries to execute the Makefile from "../.."
> relative to the crate root.
> 
> Fix this by adding a `git-src` symlink in the crate that points to the Git
> repository root. `cargo package` will flatten this to a copy of the Git repo,
> excluding non-version-controlled files, any explicitly-excluded files, and trees
> that contain a Cargo.toml file (this prevents infinite recursion on the
> symlink).
> 
> We can then execute the Makefile under the flattened git-src directory from our
> build.rs script. However, this exposes a second problem; Cargo will check that
> the build script does not add, delete, or modify any source files. This means
> that after we copy our libgitpub.a dependency to the output directory, we must
> run `make clean` to remove the object files we created during the build process.
> 
> Unfortunately, there is not a way to determine from the build.rs script whether
> we're running `cargo build` vs. `cargo package`, so now any build of the
> libgit-sys crate will result in cleaning the Git worktree.
> 
> A potential alternative is to make an additional temporary copy of the worktree
> and run the Makefile there. This would avoid removing build artifacts in the
> worktree at the cost of copying MBs worth of source files to a temporary
> directory. Perhaps hardlinking instead of making a full copy would help here,
> but that might be less portable.