oid: specify a default implementation libgit2's current default oid type is SHA1, set a public macro for that.
diff --git a/include/git2/oid.h b/include/git2/oid.h
index 924c680..6136e28 100644
--- a/include/git2/oid.h
+++ b/include/git2/oid.h
@@ -25,6 +25,9 @@ typedef enum {
GIT_OID_SHA256 = 2 /**< SHA256 */
} git_oid_t;
+/** SHA1 is currently libgit2's default oid type. */
+#define GIT_OID_DEFAULT GIT_OID_SHA1
+
/** Size (in bytes) of a raw/binary oid */
#define GIT_OID_SHA1_SIZE 20
#define GIT_OID_SHA256_SIZE 32