VimwikiGenerateTags links don't handle file hierarchy correctly

Create a tag (e.g. :SomeTag:), rebuild tags, and in diary/2020-01-02.wiki issue the command :VimwikiGenerateTags.

This results in links like [[diary/2020-01-02#SomeTag]]. When that link is followed, vim tries to create diary/diary/2020-01-02.wiki. This is wrong:
- either links should be generated based on the main wiki directory (like [[/diary/2020-01-02#SomeTag]]),
- or (better) they should be generated relative to the file in which the :VimwikiGenerateTags command is issued ([[2020-01-02#SomeTag]]).

This is a follow-on from #190.

Syntax: default.

From `.vimrc`:

let wiki = {}
let wiki.name = 'wiki'
let wiki.path = '~/wiki/'
let wiki.nested_syntaxes = nested_syntaxes
let wiki.auto_tags = 1

Branch: dev
Revision: 5cdeb9a
Date: 2020-01-01 10:58:56 -0700