Whe already worked out the kinks with the function used in the local
transport. Expose it and make use of it in the local clone method
instead of trying to work it out again.

When using Iconv to convert unicode data and iconv doesn't like
the source data (because it thinks that it's not actual UTF-8),
instead of stopping the operation, just use the unconverted data.
This will generally do the right thing on the filesystem, since
that is the source of the non-UTF-8 path data anyhow.

This adds some tests for creating and looking up branches with
messy Unicode names.  Also, this takes the helper function that
was previously internal to `git_repository_init` and makes it
into `git_path_does_fs_decompose_unicode` which is a useful in
tests to understand what the expected results should be.

When traversing the directory structure, the iterator pushes and
pops ignore files using a vector.  Some directories don't have
ignore files, so it uses a path comparison to decide when it is
right to actually pop the last ignore file.  This was only
comparing directory suffixes, though, so a subdirectory with the
same name as a parent could result in the parent's .gitignore
being popped off the list ignores too early.  This changes the
logic to compare the entire relative path of the ignore file.

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.

When a directory containing a .git directory (or even just a plain
gitlink) was found, libgit2 was going out of its way to treat it
specially.  This seemed like it was necessary because the diff
code was not originally emulating Git's behavior for untracked
directories correctly (i.e. scanning for ignored vs untracked items
inside).  Now that libgit2 diff mimics Git's untracked directory
behavior, the special handling for contained Git repos is actually
incorrect and this commit rips it out.

If a directory disappears between the time we look up the entries of its
parent and the time when we go to look at it, we should ignore the error
and move forward.

This fixes #2046.