This updates how libgit2 treats submodule-like directories that
actually have tracked content inside of them.  This is a strange
corner case, but it seems that many people have abortive submodule
setups and then just went ahead and added the files into the
parent repository.  In this case, we should just treat the
submodule as if it was a normal directory.

Libgit2 will still try to skip over real submodules and contained
repositories that do not have tracked files inside them, but this
adds some new handling for cases where the apparently submodule
data is in conflict with the actual list of tracked files.

git_merge_base() returns GIT_ENOTFOUND when it cannot find a merge
base. graph_desdendant_of() returns a boolean value (barring any
errors), so it needs to catch the NOTFOUND return value and convert it
into false, as not merge base means it cannot be a descendant.

Correct grouping of parentheses

git_graph_descendant_of was returning the result of an assignment

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.

Add support for ** matches in ignores