Handle http.* config variables pointing to files gracefully on Windows

Handle http.* config variables pointing to files gracefully on Windows

On Windows, we would like to be able to have a default http.sslCAinfo
that points to an MSys path (i.e. relative to the installation root of
Git).  As Git is a MinGW program, it has to handle the conversion
of the MSys path into a MinGW32 path itself.

Since system_path() considers paths starting with '/' as absolute, we
have to convince it to make a Windows path by stripping the leading
slash.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>