git-pickaxe: get rid of wasteful find_origin().

After finding out which path in the parent to scan to pass
blames, using get_tree_entry() to extract the blob information
again was quite wasteful, since diff-tree already gave us that
information.  Separate the function to create an origin out as
get_origin().

You'll never know what is more efficient unless you try and/or
think hard.  I somehow thought that extracting one known path
out of commit's tree is cheaper than running a diff-tree for the
current path between the commit and its parent, but it is not
the case.  In real, non-toy projects, most commits do not touch
the path you are interested in, and if the path is a few levels
away from the toplevel, whole-subdirectory comparison logic
diff-tree allows us to skip opening lower subdirectories.

This commit rewrites find_origin() function to use a single-path
diff-tree to see if the parent has the same blob as the current
suspect, which is cheaper than extracting the blob information
using get_tree_entry() and comparing it with what the current
suspect has.  This shaves about 6% overhead when annotating
kernel/sched.c in the Linux kernel repository on my machine.
The saving rises to 25% for arch/i386/kernel/Makefile.

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

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