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

Provide a convenience function that creates a buffer that can be provided
to callers but will not be freed via `git_buf_free`, so the buffer
creator maintains the allocation lifecycle of the buffer's contents.

Without this change, compiling with gcc and pedantic generates warning:
ISO C does not allow extra ‘;’ outside of a function.

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.