Update `git_win32_path_remove_namespace` to disambiguate the prefix
being removed versus the prefix being added.  Now we remove the
"namespace", and (may) add a "prefix" in its place.  Eg, we remove the
`\\?\` namespace.  We remove the `\\?\UNC\` namespace, and replace it
with the `\\` prefix.  This aids readability somewhat.

Additionally, use pointer arithmetic instead of offsets, which seems to
also help readability.