git-pickaxe: swap comparison loop used for -C

git-pickaxe: swap comparison loop used for -C

When assigning blames for code movements across file boundaries,
we used to iterate over blame entries (i.e. groups of lines to
be blamed) in the outer loop and compared each entry with paths
in the parent commit in an inner loop.  This meant that we
opened the blob data from each path number of times.

Reorganize the loop so that we read the same path only once, and
compare it against all relevant blame entries.

This should perform better, but seems to give mixed results,
though.

Signed-off-by: Junio C Hamano <junkio@cox.net>