line-range: teach -L/RE/ to search relative to anchor point

line-range: teach -L/RE/ to search relative to anchor point

Range specification -L/RE/ for blame/log unconditionally begins
searching at line one. Mailing list discussion [1] suggests that, in the
presence of multiple -L options, -L/RE/ should search relative to the
endpoint of the previous -L range, if any.

Teach the parsing machinery underlying blame's and log's -L options to
accept a start point for -L/RE/ searches. Follow-up patches will upgrade
blame and log to take advantage of this ability.

[1]: http://thread.gmane.org/gmane.comp.version-control.git/229755/focus=229966

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