Change 'Deltifying objects' to 'Compressing objects'

Change 'Deltifying objects' to 'Compressing objects'

Recently I was referred to the Grammar Police as the git-pack-objects
progress message 'Deltifying %u objects' is considered to be not
proper English to at least some small but vocal segment of the
English speaking population.  Techncially we are applying delta
compression to these objects at this stage, so the new term is
slightly more acceptable to the Grammar Police but is also just
as correct.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>