If we're looking for a symlink, realpath will give us the resolved path,
which is not what we're after, but a canonicalized version of the path
the user asked for.

The previous commit left the comment referencing the earlier state of
the code, change it to explain the current logic. While here, change the
logic to avoid repeating the copy of the base pattern.

When we discover that we want to keep a negative rule, make sure to
clear the error variable, as it we otherwise return whatever was left by
the previous loop iteration.

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

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.

A rule can only negate something which was explicitly mentioned in the
rules before it. Change our parsing to ignore a negative rule which does
not negate something mentioned in the rules above it.

While here, fix a wrong allocator usage. The memory for the match string
comes from pool allocator. We must not free it with the general
allocator. We can instead simply forget the string and it will be
cleaned up.

The diff code was using an "ignored_prefix" directory to track if
a parent directory was ignored that contained untracked files
alongside tracked files. Unfortunately, when negative ignore rules
were used for directories inside ignored parents, the wrong rules
were applied to untracked files inside the negatively ignored
child directories.

This commit moves the logic for ignore containment into the workdir
iterator (which is a better place for it), so the ignored-ness of
a directory is contained in the frame stack during traversal.  This
allows a child directory to override with a negative ignore and yet
still restore the ignored state of the parent when we traverse out
of the child.

Along with this, there are some problems with "directory only"
ignore rules on container directories.  Given "a/*" and "!a/b/c/"
(where the second rule is a directory rule but the first rule is
just a generic prefix rule), then the directory only constraint
was having "a/b/c/d/file" match the first rule and not the second.
This was fixed by having ignore directory-only rules test a rule
against the prefix of a file with LEADINGDIR enabled.

Lastly, spot checks for ignores using `git_ignore_path_is_ignored`
were tested from the top directory down to the bottom to deal with
the containment problem, but this is wrong. We have to test bottom
to top so that negative subdirectory rules will be checked before
parent ignore rules.

This does change the behavior of some existing tests, but it seems
only to bring us more in line with core Git, so I think those
changes are acceptable.