Fix bug discovered by John W. Shipman -- when the width of a format

specifier came from an int expression instead of a constant in the
format, a negative width was truncated to zero instead of taken to
mean the same as that negative constant plugged into the format.  E.g.
"(%*s)" % (-5, "foo") yielded "(foo)" while "(%-5s)" yields "(foo  )".
Now both yield the latter -- like sprintf() in C.

Objects/stringobject.c

@@ -832,8 +832,10 @@ PyString_Format(format, args)
 					goto error;
 				}
 				width = PyInt_AsLong(v);
-				if (width < 0)
-					width = 0;
+				if (width < 0) {
+					flags |= F_LJUST;
+					width = -width;
+				}
 				if (--fmtcnt >= 0)
 					c = *fmt++;
 			}