Fix the handling of .git/info/exclude and core.excludesFile
The RootIgnoreNode in a WorkingTreeIterator must _not_ add the rules
from .git/info/exclude or from the file designated by git config
core.excludesFile to the list of rules read from the root .gitignore.
These really must be separate nodes in a hierarchy, otherwise the
precedence rules from [1] are violated and the outcome is not the
same as in C git.
[1] https://git-scm.com/docs/gitignore
Bug: 580381
Change-Id: I57802ba7bbbe4f183504c882b6c77a78cc3a9b99 Signed-off-by: Thomas Wolf <twolf@apache.org>