Unlike ignore files, gitattribute files can have flexible whitespace at
the beginning of the line.  Ensure that by adding new ignore rules that
we have not impeded correct parsing of attribute files.

The checks to see if files were out of date in the attibute cache
was wrong because the cache-breaker data wasn't getting stored
correctly.  Additionally, when the cache-breaker triggered, the
old file data was being leaked.

This is a big refactoring of the attribute file cache to be a bit
simpler which in turn makes it easier to enforce a lock around any
updates to the cache so that it can be used in a threaded env.
Tons of changes to the attributes and ignores code.