Test decomposed unicode renames work as expected
Test toggling core.precomposeunicode yields rename

There is an interesting difference with core Git here, though.
Because libgit2 will do rename detection with the working directory,
in the last case where the HEAD and the working directory both
have the decomposed data and the index has the composed data, we
generate a single status record with two renames whereas Git will
generate one rename (head to index) and one untracked file.
Improve docs for status rename detection limits

and make tests empty on platforms without iconv support.
Merge pull request #2297 from libgit2/rb/status-with-precomposed-changes

Improve test coverage of status with different core.precomposeunicode settings