The workdir iterator has always tried to ignore .git files, but
it turns out there were some bugs.  This makes it more robust at
ignoring .git files.

This also makes iterators always check ".git" case insensitively
regardless of the properties of the system.  This will make libgit2
skip ".GIT" and the like.  This is different from core git, but on
systems with case insensitive but case preserving file systems,
allowing ".GIT" to be added is problematic.