Merge branch 'np/types'

* np/types:
Cleanup check_valid in commit-tree.
make sure enum object_type is signed
get rid of lookup_object_type()
convert object type handling from a string to a number
formalize typename(), and add its reverse type_from_string()
sha1_file.c: don't ignore an error condition in sha1_loose_object_info()
sha1_file.c: cleanup "offset" usage
sha1_file.c: cleanup hdr usage

diff --cc diff.c
Simple merge

diff --cc index-pack.c
Simple merge