dir: allow exclusions from blob in addition to file

dir: allow exclusions from blob in addition to file

Refactor add_excludes() to separate the reading of the
exclude file into a buffer and the parsing of the buffer
into exclude_list items.

Add add_excludes_from_blob_to_list() to allow an exclude
file be specified with an OID without assuming a local
worktree or index exists.

Refactor read_skip_worktree_file_from_index() and add
do_read_blob() to eliminate duplication of preliminary
processing of blob contents.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>