oid: add functions to inspect oid information

Provide helper functions to provide information about the object id size
given its type.

src/libgit2/oid.h

@@ -14,6 +14,26 @@
 extern const git_oid git_oid__empty_blob_sha1;
 extern const git_oid git_oid__empty_tree_sha1;
 
+GIT_INLINE(size_t) git_oid_size(git_oid_t type)
+{
+	switch (type) {
+	case GIT_OID_SHA1:
+		return GIT_OID_SHA1_SIZE;
+	}
+
+	return 0;
+}
+
+GIT_INLINE(size_t) git_oid_hexsize(git_oid_t type)
+{
+	switch (type) {
+	case GIT_OID_SHA1:
+		return GIT_OID_SHA1_HEXSIZE;
+	}
+
+	return 0;
+}
+
 /**
  * Format a git_oid into a newly allocated c-string.
  *