test: use unambigous leading path (/foo) for MSYS

test: use unambigous leading path (/foo) for MSYS

In test cases for relative_path, path with one leading character
(such as /a, /x) may be recogonized as "a:/" or "x:/" if there is
such DOS drive on MSYS platform. Use an umambigous leading path
"/foo" instead.

Also change two leading slashes (//) to three leading slashes (///),
otherwize it will be recognized as UNC name on MSYS platform.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Acked-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>