Respect core.precomposeunicode = true in more cases.

This fixes two issues I found when core.precomposeunicode is enabled:

* When creating a reference with a NFD string, the returned
git_reference would
  return this NFD string as the reference’s name. But when looking up
the reference
  later, the name would then be returned as NFC string.
* Renaming a reference would not honor the core.precomposeunicode and
apply
  no normalization to the new reference name.