Merge branch 'rs/rebase-commit-validation' into maint

Diagnose command line error of "git rebase" early.

* rs/rebase-commit-validation:
  rebase: verify commit parameter

Author: gitster

builtin/rebase.c

@@ -1917,7 +1917,9 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 			die_if_checked_out(buf.buf, 1);
 			options.head_name = xstrdup(buf.buf);
 		/* If not is it a valid ref (branch or commit)? */
-		} else if (!get_oid(branch_name, &options.orig_head))
+		} else if (!get_oid(branch_name, &options.orig_head) &&
+			   lookup_commit_reference(the_repository,
+						   &options.orig_head))
 			options.head_name = NULL;
 		else
 			die(_("fatal: no such branch/commit '%s'"),