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.