Improve test coverage of status with different core.precomposeunicode settings

This adds tests to confirm what the behavior of status will be when using decomposed and/or composed unicode strings with various values of `core.precomposeunicode` include changing the setting on an existing repository.

This also clarifies the documentation of some limitations of `git_status_file` and the `git_status` APIs when a `pathspec` setting is used w.r.t. being able to correct detect renames in status.

See https://github.com/libgit2/libgit2/issues/2293#issuecomment-41338149 for some relevant comments, including examples of how regular Git has the same limitation with detecting renames when a filename filter is applied.