ref-filter: use REF_ITERATOR_SEEK_SET_PREFIX instead of '1'

ref-filter: use REF_ITERATOR_SEEK_SET_PREFIX instead of '1'

In the commit 51511d68f4 (for-each-ref: introduce a '--start-after'
option, 2025-07-15), for introducing the '--start-after' flag, the
`ref_iterator_seek()` was modified to also accept a flag. This was to
allow the function to also set the prefix when
'REF_ITERATOR_SEEK_SET_PREFIX' was set.

In `do_filter_refs()` instead of passing the flag, we pass in '1' which
is the value of the flag. While this works, this is definitely hard to
read and introduces inconsistency. Change it to use the flag.

While here, remove the unnecessary 'if (prefix)' clause in the 'else'
statement, since the block already checks for 'prefix'.

Reported-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>