Use git rev-parse to get tag canonical revision

[huandu]

git rev-parse is preferred to get tag's canonical revision. And a side effect of this PR is to fix #112 per my test.

[sodabrew]

This makes sense. But let's also squash the if conditions together. I can submit a separate PR, but here's what I'm thinking:

    canonical = at_path { git_with_identity('rev-parse', '--revs-only', @resource.value(:revision)).chomp }
    if canonical.empty? && tag_revision?(@resource.value(:revision))
      # git rev-parse executed properly but didn't find the ref;
      # look for it in the remote
      remote_ref = at_path { git_with_identity('ls-remote', '--heads', '--tags', @resource.value(:remote), @resource.value(:revision)).chomp }
      if remote_ref.empty?
        fail("#{@resource.value(:revision)} is not a local or remote ref")
      end

      # $ git ls-remote --heads --tags origin feature/cvs
      # 7d4244b35e72904e30130cad6d2258f901c16f1a    refs/heads/feature/cvs
      canonical = remote_ref.split.first
    end

[sodabrew]

I'm not sure why we wouldn't look at the remote in case the revision is a tag.

[huandu]

@sodabrew A possible reason for your question is that tag_revision? only checks local tags. If a tag is not recognized as local tag, the else branch will be executed so that the tag will be looked at remote.

[sodabrew]

@apenney I'll submit a PR per #120 (comment) :)