Blobs that have been marked as uninteresting should not be inserted into packbuilder
when inserting a tree. The check as to whether a blob was uninteresting looked at
the status for the tree itself instead of the blob.

This could cause significantly larger packfiles.

We strive to keep an options structure to many functions to be able to
extend options in the future without breaking the API. `git_indexer_new`
doesn't have one right now, but we want to be able to add an option
for enabling strict packfile verification.

Add a new `git_indexer_options` structure and adjust callers to use
that.

The `git_walk_objects` structure is currently only being used inside of
the pack-objects.c file, but being declared in its header. This has
actually been the case since its inception in 04a36feff (pack-objects:
fill a packbuilder from a walk, 2014-10-11) and has never really
changed.

Move the struct declaration into pack-objects.c to improve code
encapsulation.

Transfer fewer objects on push and local fetch