- Fix license to be GPL-2.0-with-GCC-exception

- Update to 0.16.0
- Git Attributes support (see git2/attr.h)
There is now support to efficiently parse and retrieve information
from `.gitattribute` files in a repository. Note that this
information is not yet used e.g. when checking out files.
- .gitignore support
Likewise, all the operations that are affected by `.gitignore` files
now take into account the global, user and local ignores when
skipping the relevant files.
- Cleanup of the object ownership semantics
The ownership semantics for all repository subparts (index, odb,
config files, etc) has been redesigned. All these objects are now
reference counted, and can be hot-swapped in the middle of
execution, allowing for instance to add a working directory and an
index to a repository that was previously opened as bare, or to
change the source of the ODB objects after initialization.
Consequently, the repository API has been simplified to remove all
the `_openX` calls that allowed setting these subparts *before*
initialization.
- git_index_read_tree()
Git trees can now be read into the index.
- More reflog functionality
The reference log has been optimized, and new API calls to rename
and delete the logs for a reference have been added.
- Rewrite of the References code with explicit ownership semantics
The references code has been mostly rewritten to take into account
the cases where another Git application was modifying a repository's
references while the Library was running.