dir: create function count_slashes()

dir: create function count_slashes()

Similar functions exist in apply.c and builtin/show-branch.c for
counting the number of slashes in a string. Also in the later
patches, we introduce a third caller for the same. Hence, we unify
it now by cleaning the existing functions and declaring a common
function count_slashes in dir.h and implementing it in dir.c to
remove this code duplication.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Prathamesh Chavan <pc44800@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>