Re: [RFC PATCH] diff: do not use creation-half of -B as a rename target candidate

[Junio C Hamano <gitster@pobox.com>]

Junio C Hamano <gitster@pobox.com> writes:

>  * Here is what I am at the moment; I cannot quite explain (hence I
>    cannot convince myself) why this is the right solution, but it
>    seems to make the above sample case work without breaking any
>    existing tests.  It is possible that the tests that would break
>    without the "&& !p->score" bit are expecting wrong results, but I
>    didn't look at them in detail.

Sadly, I think this is garbage.  "Do not consider creation-half of a
broken pair, ever" is too simple and cripples this case that starts
with two files A and B that are quite different:

	$ git add A B
	$ mv A B.new
        $ mv B A
        $ mv B.new B
        $ git diff -B -M

where the internal machinery breaks both A and B into these two file
pairs:

	delete A(old)
        create A(new)

	delete B(old)
        create B(new)

and then match them up to produce

	rename A to B
        rename B to A

The rule need to be "creation-half of a broken pair can be used as
the destination of a rename, if and only if its corresponding
deletion-half is used as the source of another rename elsewhere".
Under that condition, a file A that is completely rewritten to
become similar to another existing file B can be expressed as a
rename of B, because A is renamed away to make room in the same
change.

Fixing this is turning out to be more complex than I originally
hoped X-<.