Skip ignored directories in FileTreeIterator

Skip ignored directories in FileTreeIterator

Make FileTreeIterator not enter ignored directories by default. We
only need to enter ignored directories if we do some operation against
git, and there is at least one tracked file underneath an ignored
directory.

Walking ignored directories should be avoided as much as possible as
it is a potential performance bottleneck. Some projects have a lot of
files or very deep hierarchies in ignored directories; walking those
may be costly (especially so on Windows). See for instance also bug
500106.

Provide a FileTreeIterator.setWalkIgnoredDirectories() operation to
force the iterator to iterate also through otherwise ignored
directories. Useful for tests (IgnoreNodeTest, CGitIgnoreTest), or
to implement things like "git ls-files --ignored".

Add tests in DirCacheCheckoutTest, and amend IndexDiffTest to test a
little bit more.

Bug: 388582
Change-Id: I6ff584a42c55a07120a4369fd308409431bdb94a
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>