config: normalize the path of the system gitconfig

config: normalize the path of the system gitconfig

Git for Windows is compiled with a runtime prefix, and that runtime
prefix is typically `C:/Program Files/Git/mingw64`. As we want the
system gitconfig to live in the sibling directory `etc`, we define the
relative path as `../etc/gitconfig`.

However, as reported by Philip Oakley, the output of `git config
--show-origin --system -l` looks rather ugly, as it shows the path as
`file:C:/Program Files/Git/mingw64/../etc/gitconfig`, i.e. with the
`mingw64/../` part.

By normalizing the path, we get a prettier path.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>