setup_git_directory(): use is_dir_sep() helper

setup_git_directory(): use is_dir_sep() helper

It is okay in practice to test for forward slashes in the output of
getcwd(), because we go out of our way to convert backslashes to forward
slashes in getcwd()'s output on Windows.

Still, the correct way to test for a dir separator is by using the
helper function we introduced for that very purpose. It also serves as a
good documentation what the code tries to do (not "how").

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