During checkout, assume that the .gitattributes files aren't
modified during the checkout. Instead, create an "attribute session"
during checkout. Assume that attribute data read in the same
checkout "session" hasn't been modified since the checkout started.
(But allow subsequent checkouts to invalidate the cache.)
Further, cache nonexistent git_attr_file data even when .gitattributes
files are not found to prevent re-scanning for nonexistent 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The checks to see if files were out of date in the attibute cache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.
