### Please describe the problem.

A `git-annex get` fails with `createDirectory: does not exist (No such file or directory)` on MacOSX.

### What steps will reproduce the problem?

I can trigger the condition as part of a CI run of the DataLad test suite. Here is an example
run that shows the failure: https://ci.appveyor.com/project/mih/datalad/build/job/k5u263619e6erk8t
However, the exact conditions required to trigger the issue are not yet known (c.f. https://github.com/datalad/datalad/issues/5291).

A protocol of an exploration of this issue with debug output is here: https://github.com/datalad/datalad/issues/5301#issuecomment-757467813

In this end I can trigger the error with a `mkdir` performed manually in the shell using the path reported by git-annex (e.g. `../../../../../../../var/folders/5s/g225f6nd6jl4g8tshbh1ltk40000gn/T/datalad_temp_tree_test_basic_scenariodi3ady04/.git/annex/`). But a `mkdir` is successful using a "normalized variant of the path
pointing to the same physical directory (e.g. `/var/folders/5s/g225f6nd6jl4g8tshbh1ltk40000gn/T/datalad_temp_tree_test_basic_scenariodi3ady04/.git/annex`).

I do not understand enough of this platform to understand what is happening, but it seems that some of these folders actually live in `/private/var`, despite being accessible via `/var`, but I do not see how `mkdir` would error on a relative path and succeed on an absolute one.


### What version of git-annex are you using? On what operating system?

8.20201129 on darwin/19.6.0 10.15.7/x86_64

More system details are here https://ci.appveyor.com/project/mih/datalad/build/job/k5u263619e6erk8t#L422


### Have you had any luck using git-annex before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)

git-annex is critical infrastructure for me. There is no day without it. Thx much!


[[!tag projects/datalad]]

> So there's an OSX bug here, and git-annex has been made to use
> an absolute path to a remote when it has one, which can be used to work
> around the OSX bug. [[done]] --[[Joey]]
