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

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

This is complicated slightly by having to remember the previous -L range
for each file specified via -L<range>:file.

The existing implementation coalesces ranges for each file as each -L is
parsed which makes it impossible to refer back to the previous -L range
for any particular file. Re-implement to instead store each file's set
of -L ranges verbatim, and then coalesce the ranges in a post-processing
step.

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