[BUG?] read_directory(), core.ignorecase and pathspec

I was looking at how read_directory() uses pathspec, and it seems that simplify_away(), the function that cuts out directories early if they're definitely outside given pathspec, does exact match (ie. memcmp) regardless core.ignorecase. This means "git add -- '*.c'" may not work as expected when core.ignorecase is on.

I haven't tested it though. Just heads up. If you care about core.ignorecase (I don't apparently), you can try it out.

The whole simplifying thing in read_directory() will eventually be replaced with real pathspec matching as we put more magic in pathspec.