We may need to strip multiple slashes at the end of a path; provide a
method to do so.

We may quickly want to determine if the given path is the root path
('/') on POSIX, or the root of a drive letter (eg, 'A:/', 'C:\') on
Windows.

Clar handles multiple levels of hierarchy in a test name _but_ it does so
assuming that there are not tests at a parent folder level. In other words,
if you have some tests at path/core.c and path/win32.c, then you cannot have
tests in path.c. If you have tests in path.c, then the things in path/*.c
will be ignored.

Move the tests in path.c into path/core.c.

Ensure that we clean up cruft that we create for testing, so that future
tests don't have troubles.

Our error messages should provide the literal path that users should add
to their safe directory allowlist, which means it should not have a
trailing slash.

Ensure that we can support safe.directory entries that are prefixed with
'%(prefix)/'.

'%(prefix)/' as a leading path name is safe to strip on all platforms.
It doesn't make sense to use on non-Windows, but supporting it there
does allow us to easily dev/test.

Provide a helper method in the tests to set up the safe.directory
configuration for a normal and bare repository to avoid some copy/pasta.

(Some copypasta will remain, since there's customizations to the file
that are not trivially abstractable.)

On Windows, you may open a repo with UNC path format -- for example
\\share\foo\bar, or as a git-style path, //share/foo/bar. In this world,
Git for Windows expects you to translate this to
$(prefix)//share/foo/bar.

We can test for that by using the fact that local drives are exposed as
hidden shares. For example, C:\Foo is //localhost/C$/Foo/.

When doing directory name munging to remove trailing slashes, ensure
that we do not remove the trailing slash from the path root, whether
that's '/' or 'C:\'.

Recent versions of Git for Windows allow a sane UNC style path to be
used directly in `safe.directory` (without needing the `%(prefix)`
silliness). Match that behavior.