am: change safe_to_abort()'s not rewinding error into a warning

The error message tells the user that something went terribly wrong
and the --abort could not be performed. But the --abort is performed,
only without rewinding. By simply changing the error into a warning,
we indicate the user that she must not try something like
"git am --abort --force", instead she just has to check the HEAD.

Signed-off-by: Stephan Beyer <s-beyer@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/am.c

@@ -2134,7 +2134,7 @@ static int safe_to_abort(const struct am_state *state)
 	if (!oidcmp(&head, &abort_safety))
 		return 1;
 
-	error(_("You seem to have moved HEAD since the last 'am' failure.\n"
+	warning(_("You seem to have moved HEAD since the last 'am' failure.\n"
 		"Not rewinding to ORIG_HEAD"));
 
 	return 0;