fix warning with libgit2 v0.99+, remain compatible with older versions

fix warning with libgit2 v0.99+, remain compatible with older versions

git_blob_rawsize now returns with git_object_size_t (unsigned). This was
git_off_t (signed).

In my current version 1.1.0:
	types.h:typedef uint64_t git_object_size_t;

v0.28.5:
https://libgit2.org/libgit2/#v0.28.5/group/blob/git_blob_rawsize

changed from v0.99 onwards:
https://libgit2.org/libgit2/#v0.99.0/group/blob/git_blob_rawsize

Fix: use size_t to remain compatible (with a possible warning in older
versions), since git_object_size_t is a new defined type.
This assumes size_t is atleast uint32_t / uint64_t size.

Adapted from a patch by Augustin Fabre <augustin@augfab.fr>, thanks!