relative_path should honor dos-drive-prefix

relative_path should honor dos-drive-prefix

Tvangeste found that the "relative_path" function could not work
properly on Windows if "in" and "prefix" have DOS drive prefix
(such as "C:/windows"). ($gmane/234434)

E.g., When execute: test-path-utils relative_path "C:/a/b" "D:/x/y",
should return "C:/a/b", but returns "../../C:/a/b", which is wrong.

So make relative_path honor DOS drive prefix, and add test cases
for it in t0060.

Reported-by: Tvangeste <redacted>
Helped-by: Johannes Sixt <redacted>
Signed-off-by: Jiang Xin <redacted>
Signed-off-by: Jonathan Nieder <redacted>