safe_create_leading_directories(): on Windows, \ can separate path components

safe_create_leading_directories(): on Windows, \ can separate path components

When cloning to a directory "C:\foo\bar" from Windows' cmd.exe where
"foo" does not exist yet, Git would throw an error like

    fatal: could not create work tree dir 'c:\foo\bar'.: No such file or directory

Fix this by not hard-coding a platform specific directory separator
into safe_create_leading_directories().

This patch, including its entire commit message, is derived from a
patch by Sebastian Schuberth.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>