Minimizing the number directory and file opens, minimizes the amount of IO thus reducing the overall cost of performing ignore operations.

Closes #2966.

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.

That's a bad assumption to make, even though right now it holds
(because of the way we've implemented decompression of packfiles),
this may change in the future, given that ODB objects can be
binary data.

Furthermore, the ODB object can return a NULL pointer if the object
is empty. Copying the NULL pointer to the strbuf lets us handle it
like an empty string. Again, the NULL pointer is valid behavior because
you're supposed to check the *size* of the object before working
on it.

A rule "src" in src/.gitignore must only match subdirectories of
src/. The current code does not include this context in the match rule
and would thus consider this rule to match the top-level src/ directory
instead of the intended src/src/.

Keep track fo the context in which the rule was defined so we can
perform a prefix match.

We currently consider CR to start the end of the line, but that means
that we miss cases with CR CR LF which can be used with git to match
files whose names have CR at the end of their names.

The fix from the patch comes from Russell's comment in the issue.

This fixes #2536.