dir.c: rename cryptic 'which' variable to more consistent name

'el' is only *slightly* less cryptic, but is already used as the
variable name for a struct exclude_list pointer in numerous other
places, so this reduces the number of cryptic variable names in use by
one :-)

Signed-off-by: Adam Spiers <git@adamspiers.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>