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 <i.4m.l33t@yandex.ru>
Helped-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
diff --git a/t/t0060-path-utils.sh b/t/t0060-path-utils.sh
index 92976e0..40dfa2d 100755
--- a/t/t0060-path-utils.sh
+++ b/t/t0060-path-utils.sh
@@ -210,6 +210,10 @@
 relative_path foo/a		foo/a/b		../
 relative_path foo/x/y		foo/a/b		../../x/y
 relative_path foo/a/c		foo/a/b		../c
+relative_path foo/a/b		/foo/x/y	foo/a/b
+relative_path /foo/a/b		foo/x/y		/foo/a/b
+relative_path d:/a/b		D:/a/c		../b		MINGW
+relative_path C:/a/b		D:/a/c		C:/a/b		MINGW
 relative_path foo/a/b		"<empty>"	foo/a/b
 relative_path foo/a/b 		"<null>"	foo/a/b
 relative_path "<empty>"		/foo/a/b	./