notes: do not break note_tree structure in note_tree_consolidate()

After a note is removed, note_tree_consolidate is called to eliminate
some useless nodes. The typical case is that if you had an int_node
with 2 PTR_TYPE_NOTEs in it, and remove one of them, then the
PTR_TYPE_INTERNAL pointer in the parent tree can be replaced with the
remaining PTR_TYPE_NOTE.

This works fine when PTR_TYPE_NOTEs are involved, but falls flat when
other types are involved.

To put things in more practical terms, let's say we start from an empty
notes tree, and add 3 notes:

- one for a sha1 that starts with 424
- one for a sha1 that starts with 428
- one for a sha1 that starts with 4c

To keep track of this, note_tree.root will have a PTR_TYPE_INTERNAL at
a[4], pointing to an int_node*.
In turn, that int_node* will have a PTR_TYPE_NOTE at a[0xc], pointing to
the leaf_node* with the key and value, and a PTR_TYPE_INTERNAL at a[2],
pointing to another int_node*.
That other int_node* will have 2 PTR_TYPE_NOTE, one at a[4] and the
other at a[8].

When looking for the note for the sha1 starting with 428, get_note() will
recurse through (simplified) root.a[4].a[2].a[8].

Now, if we remove the note for the sha1 that starts with 4c, we're left
with a int_node* with only one PTR_TYPE_INTERNAL entry in it. After
note_tree_consolidate runs, root.a[4] now points to what used to be
pointed at by root.a[4].a[2].

Which means looking up for the note for the sha1 starting with 428 now
fails because there is nothing at root.a[4].a[2] anymore: there is only
root.a[4].a[4] and root.a[4].a[8], which don't match the expected
structure for the lookup.

So if all there is left in an int_node* is a PTR_TYPE_INTERNAL pointer,
we can't safely remove it. I think the same applies for PTR_TYPE_SUBTREE
pointers. IOW, only PTR_TYPE_NOTEs are safe to be moved to the parent
int_node*.

This doesn't have a practical effect on git because all that happens
after a remove_note is a write_notes_tree, which just iterates the entire
note tree, but this affects anything using libgit.a that would try to do
lookups after removing notes.

Signed-off-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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