pack: move approximate object count to object store

pack: move approximate object count to object store

The approximate_object_count() function maintains a rough count of
objects in a repository to estimate how long object name abbreviates
should be.  Object names are scoped to a repository and the
appropriate length may differ by repository, so the object count
should not be global.

Signed-off-by: Stefan Beller <redacted>
Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
Signed-off-by: Junio C Hamano <redacted>