Simplify git_path_is_empty_dir implementation

This simplifies git_path_is_empty_dir on both Windows (getting rid
of git_buf allocation inside the function) and other platforms (by
just using git_path_direach), and adds tests for the function, and
uses the function to simplify some existing tests.
Wrap iconv stuff and write tests

This adds a simple wrapper around the iconv APIs and uses it
instead of the old code that was inlining the iconv stuff.  This
makes it possible for me to test the iconv logic in isolation.
A "no iconv" version of the API was defined with macros so that
I could have fewer ifdefs in the code itself.