add commit hash information to registry

[simonbyrne]

First part of JuliaLang/Pkg.jl#3718.

This adds 4 optional keys to each version:

• git-commit-sha1: the hash of a git commit object corresponding to the release. This is required if any of the other keys exist.
• git-tag-name: the name of the git tag
• subdir: the subdirectory of the tree with respect to the root of the repository.

These should satisfy the following:

• the tree at subdir of the commit git-commit-sha1 should be git-tree-sha1. In other words, git rev-parse $(git-commit-sha1):$(subdir) == $(git-tree-sha1). If subdir is not defined, then it should be the root tree.
• the tag git-tag-name should resolve to the commit git-commit-sha1. That is, git rev-parse $(git-tag-name)^{commit} == $(git-commit-sha1)

Unlike git-tree-sha1, these keys should all be considered potentially mutable (i.e. the repository may be modified so that the values of these keys may change).

cc @IanButterworth

[DilumAluthge]

Instead of sticking everything at the same level as git-tree-sha, what if we have a sub-dict for all optional metadata, e.g.

versions = Dict()
versions["1.2.3"] = Dict(
    "git-tree-sha1" => "...",
    "optional" => Dict(
        "git-commit-sha1" => "...",
        "some-other-info" => "helloworld",
    ),
)

[simonbyrne]

that seems more complicated?

[simonbyrne]

I decided against including git-tag-sha1, as it doesn't really add anything.