types: introduce `git_object_size_t`

Introduce `git_object_size_t`, an unsigned type that we can use for the
maximum size of git objects.

include/git2/object.h

@@ -21,6 +21,8 @@
  */
 GIT_BEGIN_DECL
 
+#define GIT_OBJECT_SIZE_MAX UINT64_MAX
+
 /**
  * Lookup a reference to one of the objects in a repository.
  *

include/git2/types.h

@@ -63,6 +63,9 @@ typedef int64_t git_time_t; /**< time in seconds from epoch */
 
 #endif
 
+/** The maximum size of an object */
+typedef uint64_t git_object_size_t;
+
 #include "buffer.h"
 #include "oid.h"