Fix bug with multiple iconv conversions in one dir

The internal buffer in the `git_path_iconv_t` structure was not
being reset before the calls to `iconv` were made to convert data,
so if there were multiple decomposed Unicode paths in a single
directory, paths after the first one were being appended to the
first instead of treated as independent data.
vmg is always right
Merge pull request #2255 from libgit2/rb/fix-multiple-nfd-iconv-bug

Fix bug with multiple iconv conversions in one dir