[RFC] use typechange as rename source

Today in one of my repositories I did something like this:

  $ mv foo bar
  $ ln -s bar foo
  $ git add .

and I expected git-status to claim:

  typechange: foo
  renamed:    foo -> bar

but it didn't find the rename (without -C) because the path 'foo' still
exists. So there is a disconnect in what git and I think of as "exists".
Should typechanges make a file eligible as a rename src?

A quickie patch to implement this is:

diff --git a/diffcore-rename.c b/diffcore-rename.c
index f9ebea5..5a34e8a 100644
--- a/diffcore-rename.c
+++ b/diffcore-rename.c

@@ -417,6 +417,10 @@ void diffcore_rename(struct diff_options *options)
 				p->one->rename_used++;
 			register_rename_src(p->one, p->score);
 		}
+		else if (DIFF_PAIR_TYPE_CHANGED(p)) {
+			p->one->rename_used++;
+			register_rename_src(p->one, p->score);
+		}
 		else if (detect_rename == DIFF_DETECT_COPY) {
 			/*
 			 * Increment the "rename_used" score by

There are a few add-on questions:

  - should typechanges in both directions be used, or just file ->
    symlink?

  - this actually produces a 'copied' status rather than a 'renamed'
    since the 'foo' entry does still exist. Is this reasonable?

-Peff