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
Correct grouping of parentheses

git_graph_descendant_of was returning the result of an assignment
Added a no path test for git_graph_descendant_of
Merge pull request #2256 from jacquesg/graph-descendant

Correct grouping of parentheses