There were quite a few places were spaces were being used instead of
tabs. Try to catch them all. This should hopefully not break anything.
Except for `git blame`. Oh well.
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.
Implemented find_unique_short_oid for pack backend, based on git sha1 lookup method;
finding an object given its full oid is just a particular case of searching
the unique object matching an oid prefix (short oid).
Added git_odb_read_unique_short_oid, which iterates over all the backends to
find and read the unique object matching the given oid prefix.
Added a git_object_lookup_short_oid method to find the unique object in
the repository matching a given oid prefix : it generalizes git_object_lookup
which now does nothing but calls git_object_lookup_short_oid.
