path.c: simplify adjust_shared_perm()

path.c: simplify adjust_shared_perm()

All calls to set_shared_perm() use mode == 0, so simplify the
function.

Because all callers use the macro adjust_shared_perm(path) from
cache.h to call this function, convert it to a proper function,
losing set_shared_perm().

Since path.c has much more functions than just mkpath() these days,
drop the stale comment about it.

Signed-off-by: Torsten Bögershausen <redacted>
Signed-off-by: Junio C Hamano <redacted>