z_stream.next_in is non-const. Although currently Zlib doesn't modify
buffer content on deflate(), it might be change in the future. gzwrite()
already modify it.

To avoid this let's change signature of git_filebuf.write and rework
git_filebuf_write() accordingly.

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>