Windows: Fix reversion that caused the path used to link to annexed
content include the drive letter and full path, rather than being
relative. (`git annex fix` will fix up after this problem).
I've not identified the commit that brought the reversion (probably it
happened this spring when I was removing MisingH and last touched
Utility.Path). Likely commit 18b9a4b8024115db67ae309fdaf54e1553037529?
The problem is that relPathDirToFile got called two paths that had the
slashes different ways around. Since takeDrive includes the first slash,
this made two paths on the same drive seem different and it bailed.
(ifdefs around this to avoid doing extra work on non-windows)
This commit was sponsored by Jack Hill on Patreon.
