object-file: move logic to compute packed abbreviation length

object-file: move logic to compute packed abbreviation length

Same as the preceding commit, move the logic that computes the minimum
required prefix length to make a given object ID unique for the packfile
store into a new function `packfile_store_find_abbrev_len()` that is
part of "packfile.c". This prepares for making the logic fully generic
via pluggable object databases.

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