Blob and tree gfm links to anchors work.

Fixes #6046
**Intended Behavior**

When on a **tree** directory such as `path/name/tree/branch/dir`, in the rendered `README.md`:

`[0]()`           | `path/name/tree/branch/dir`             | current behavior.
`[1](#id)`        | `path/name/tree/branch/dir#id`          | new. Before opened wiki.
`[2](file.md#id)` | `path/name/blob/branch/dir/file.md#id`  | new

When on a markdown **blob** such as `path/name/blob/branch/dir/file.md`:

`[0]()`            | `path/name/blob/branch/dir/file.md`     | current
`[1](#id)`         | `path/name/blob/branch/dir/file.md#id`  | new
`[2](file2.md#id)` | `path/name/blob/branch/dir/file2.md#id` | new

This differs slightly from GitHub behavior, in which `[empty]()` always points to a non existent URL.

I feel that this behavior makes more sense, because if `"#a"` points to the current file, then `""` should too.

The following is the diff for the tests under `spec/seed_project.tar.gz`. Only the `markdown` branch was modified with a new commit added. Rationale is explianed on the comments under `features/steps/project/project_markdown_render.rb`

    diff --git a/README.md b/README.md
    index 7577fc5..96c09cf 100644
    --- a/README.md
    +++ b/README.md
    @@ -1,8 +1,11 @@
    +This branch is meant to be used with for markdown tests.
    +
    +Do not rely on its commit message or SHA being constant.
    +
    # Welcome to GitLab [![build status](https://secure.travis-ci.org/gitlabhq/gitlabhq.png)](https://secure.travis-ci.org/gitlabhq/gitlabhq)
    
    GitLab is a free project and repository management application
    
    -
    ## Application details
    
    * rails 3.1
    @@ -53,3 +56,15 @@ Email
    
    Want to help - send a pull request.
    We'll accept good pull requests.
    +
    +# Links with IDs
    +
    +[empty]()
    +
    +[#id](#id)
    +
    +[/#id](/#id)
    +
    +[README.md#id](README.md#id)
    +
    +[d/README.md#id](db/README.md#id)
    diff --git a/d/README.md b/d/README.md
    new file mode 100644
    index 0000000..78addd3
    --- /dev/null
    +++ b/d/README.md
    @@ -0,0 +1,9 @@
    +[empty]()
    +
    +[#id](#id)
    +
    +[/#id](/#id)
    +
    +[README.md#id](README.md#id)
    +
    +[d/README.md#id](db/README.md#id)