pack-bitmap: deduplicate logic to iterate over preferred bitmap tips

pack-bitmap: deduplicate logic to iterate over preferred bitmap tips

We have two locations that iterate over the preferred bitmap tips as
configured by the user via "pack.preferBitmapTips". Both of these
callsites are subtly wrong: when the preferred bitmap tips contain an
exact refname match, then we will hit a `BUG()`.

Prepare for the fix by unifying the two callsites into a new
`for_each_preferred_bitmap_tip()` function.

This removes the last callsite of `bitmap_preferred_tips()` outside of
"pack-bitmap.c". As such, convert the function to be local to that file
only. Note that the function is still used by a second caller, so we
cannot just inline it.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>