Re: [PATCH] Make --follow support --find-copies-harder.

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

Bo Yang <struggleyb.nku@gmail.com> writes:

> 'git diff --follow <commit1> <commit2> <path>' give users
> the content difference of <path> between the two commits.
> It will detect file copies/moves of <path> if there is any.

Because the "--follow" hack was done primarily as a "checkbox" item, and
also because it is not an option for the "diff" family (it is an option
for the "log" family), I would personally think that it is actually a bug
that "git diff" accepts "--follow" and pretends as if it is doing useful
work, but does so only some of the time.

    $ git diff --follow --name-status maint master -- builtin/log.c
    R089	builtin-log.c	builtin/log.c
    $ git diff --follow --name-status -R maint master -- builtin/log.c
    D	builtin/log.c
    $ git diff --follow --name-status master maint -- builtin/log.c
    D	builtin/log.c

As we can see, it doesn't quite work, and it is not a fault of 750f7b6
(Finally implement "git log --follow", 2007-06-19) by Linus, exactly
because the feature wasn't designed to work with "diff" to begin with.

If we were to add a support of "--follow" to "diff" family, I suspect that
we need to

 (1) make sure we get only one path, just like "log" family does;

 (2) add a logic to notice the reverse situation as demonstrated above and
     deal with it in a sensible way, without any --find-copies option
     given by the user.

among other things.  Also we of course need to document it as a new "diff"
option when we are done.