refs: don't loose info on resolve error

Typical use is git_reference_resolve(&ref, ref). Currently, if there is
an error, ref will point to NULL, causing the user to lose that
reference.

Always update resolved_ref instead of just on finding an OID ref,
storing the last valid reference in it.

This change helps simplify the code for allowing root commits.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>

src/refs.c

@@ -1480,8 +1480,9 @@ int git_reference_resolve(git_reference **resolved_ref, git_reference *ref)
 	for (i = 0; i < MAX_NESTING_LEVEL; ++i) {
 		reference_symbolic *ref_sym;
 
+		*resolved_ref = ref;
+
 		if (ref->type & GIT_REF_OID) {
-			*resolved_ref = ref;
 			return GIT_SUCCESS;
 		}