real_path: do not assume '/' is the path seperator

real_path: do not assume '/' is the path seperator

real_path currently assumes it's input had '/' as path seperator.
This assumption does not hold true for the code-path from
prefix_path (on Windows), where real_path can be called before
normalize_path_copy.

Fix real_path so it doesn't make this assumption. Create a helper
function to reverse-search for the last path-seperator in a string.

Signed-off-by: Theo Niessink <redacted>
Signed-off-by: Erik Faye-Lund <redacted>
Signed-off-by: Junio C Hamano <redacted>