This changes the get_entry() method to return a refcounted version of
the config entry, which you have to free when you're done.
This allows us to avoid freeing the memory in which the entry is stored
on a refresh, which may happen at any time for a live config.
For this reason, get_string() has been forbidden on live configs and a
new function get_string_buf() has been added, which stores the string in
a git_buf which the user then owns.
The functions which parse the string value takea advantage of the
borrowing to parse safely and then release the entry.
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The checks to see if files were out of date in the attibute cache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.
