When trying to ensure that long paths are handled correctly, we
first normalize absolute paths as we encounter them.
However, if the path is a so-called "drive-less" absolute path, i.e. if
it is relative to the current drive but _does_ start with a directory
separator, we would want the normalized path to be such a drive-less
absolute path, too.
Let's do that, being careful to still include the drive prefix when we
need to go through the `\\?\` dance (because there, the drive prefix is
absolutely required).
This fixes https://github.com/git-for-windows/git/issues/4586.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
cec282001c