line-range: teach -L:RE to search from end of previous -L range

line-range: teach -L:RE to search from end of previous -L range

For consistency with -L/RE/, teach -L:RE to search relative to the end
of the previous -L range, if any.

The new behavior invalidates one test in t4211 which assumes that -L:RE
begins searching at start of file. This test will be resurrected in a
follow-up patch which teaches -L:RE how to override the default relative
search behavior.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>