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>
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>
Use simpler relative_path when set_git_dir

Using a relative_path as git_dir first appears in v1.5.6-1-g044bbbc.
It will make git_dir shorter only if git_dir is inside work_tree,
and this will increase performance. But my last refactor effort on
relative_path function (commit v1.8.3-rc2-12-ge02ca72) changed that.
Always use relative_path as git_dir may bring troubles like
$gmane/234434.

Because new relative_path is a combination of original relative_path
from path.c and original path_relative from quote.c, so in order to
restore the origin implementation, save the original relative_path
as remove_leading_path, and call it in setup.c.

Suggested-by: Karsten Blees <karsten.blees@gmail.com>
Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Merge remote-tracking branch 'junio/jx/relative-path-regression-fix'