1. The license header is technically not valid if it doesn't have a
copyright signature.
2. The COPYING file has been updated with the different licenses used in
the project.
3. The full GPLv2 header in each file annoys me.
The `hashfile` function has been moved to ODB, next to `git_odb_hash`.
Global state has been removed from the dirent call in `status.c`,
because global state is killing the rainforest and causing global
warming.
Add git_status_hashfile() to get blob's object id for a file without adding
it to the object database or needing a repository at all.
This functionality is similar to `git hash-object` without '-w'.