tree: return `size_t` for treebuilder entrycount

We keep the treebuilder entrycount as a `size_t` - return that instead
of downcasting to an `unsigned int`.  Callers who were storing this
value in an `unsigned int` will continue to downcast themselves, so
there should be no behavior change for callers.

include/git2/tree.h

@@ -267,7 +267,7 @@ GIT_EXTERN(void) git_treebuilder_clear(git_treebuilder *bld);
  * @param bld a previously loaded treebuilder.
  * @return the number of entries in the treebuilder
  */
-GIT_EXTERN(unsigned int) git_treebuilder_entrycount(git_treebuilder *bld);
+GIT_EXTERN(size_t) git_treebuilder_entrycount(git_treebuilder *bld);
 
 /**
  * Free a tree builder

src/tree.c

@@ -340,7 +340,7 @@ size_t git_tree_entrycount(const git_tree *tree)
 	return tree->entries.size;
 }
 
-unsigned int git_treebuilder_entrycount(git_treebuilder *bld)
+size_t git_treebuilder_entrycount(git_treebuilder *bld)
 {
 	assert(bld);