This updates all the `foreach()` type functions across the library
that take callbacks from the user to have a consistent behavior.
The rules are:

* A callback terminates the loop by returning any non-zero value
* Once the callback returns non-zero, it will not be called again
  (i.e. the loop stops all iteration regardless of state)
* If the callback returns non-zero, the parent fn returns GIT_EUSER
* Although the parent returns GIT_EUSER, no error will be set in
  the library and `giterr_last()` will return NULL if called.

This commit makes those changes across the library and adds tests
for most of the iteration APIs to make sure that they follow the
above rules.

 - Do not create new levels of fanout when creating notes from libgit2
 - Insert a note in an existing matching fanout
 - Remove a note from an existing fanout
 - Cleanup git_note_read, git_note_remove, git_note_foreach, git_note_create methods in order use tree structures instead of tree_oids

Add git_note_default_ref to allow easy retrieval of the currently set
default notes reference.

Setting core.notesRef allows to change the default notes reference used
by Git. Check if set before using GIT_NOTES_DEFAULT_REF. Fixes #649.