Different diff strategies in add --interactive

[John Keeping <john@keeping.me.uk>]

I've just been trying to use "add -p" to stage some changes which happen
to be textually entangled with other changes that I do not want to
stage.

It turns out that "git diff --patience" does a really good job
splitting this into exactly the hunks I want, but "add --interactive"
doesn't let me change the diff algorithm it uses.  I tried setting
"diff.algorithm" to "patience", but of course add--interactive uses
plumbing diff commands that ignore configuration settings.

As a one off, I locally modified add--interactive to unconditionally use
patience diff and it has worked perfectly in this case, but I don't want
to have to apply a patch if I ever want this behaviour in the future.

I think the first thing to do is read the "diff.algorithm" setting in
git-add--interactive and pass its value to the underlying diff-index and
diff-files commands, but should we also have a command line parameter to
git-add to specify the diff algorithm in interactive mode?  And if so,
can we simply add "--diff-algorithm" to git-add, or is that too
confusing?