In commit 45f24e78 (git_repository_init: stop traversing at
windows root, 2019-04-12), we have fixed `git_futils_mkdir` to
correctly handle the case where we create a directory in
Windows-style filesystem roots like "C:\repo".

The problem here is an off-by-one: previously, to that commit,
we've been checking wether the parent directory's length is equal
to the root directory's length incremented by one. When we call
the function with "/example", then the parent directory's length
("/") is 1, but the root directory offset is 0 as the path is
directly rooted without a drive prefix. This resulted in `1 == 0 +
1`, which was true. With the change, we've stopped incrementing
the root directory length, and thus now compare `1 <= 0`, which
is false.

The previous way of doing it was kind of finicky any non-obvious,
which is also why the error was introduced. So instead of just
re-adding the increment, let's explicitly add a condition that
aborts finding the parent if the current parent path is "/".

Making this change causes Azure Pipelines to fail the testcase
repo::init::nonexistent_paths on Unix-based systems. This is because we
have just fixed creating directories in the filesystem root, which
previously didn't work. As Docker-based tests are running as root user,
we are thus able to create the non-existing path and will now succeed to
create the repository that was expected to actually fail.

Let's split this up into three different tests:

- A test to verify that we do not create repos in a non-existing parent
  directoy if the flag `GIT_REPOSITORY_INIT_MKPATH` is not set.

- A test to verify that we fail if the root directory does not exist. As
  there is a common root directory on Unix-based systems that always
  exist, we can only test for this on Windows-based systems.

- A test to verify that we fail if trying to create a repository in an
  unwriteable parent directory. We can only test this if not running
  tests as root user, as CAP_DAC_OVERRIDE will cause us to ignore
  permissions when creating files.