Clarify WorkingTreeOptions and filemode usage

Clarify WorkingTreeOptions and filemode usage

To improve runtime performance, caching the WorkingTreeOptions inside
of the Config object using the Config.SectionParser API allows
the WorkingTreeOptions to be accessed more efficiently whenever a
FileTreeIterator is constructed for the Repository.

Instead of passing the filemode handling option into isModified(),
the WorkingTreeIterator should always honor whatever setting has
been configured in this repository, as defined by its own copy of
the WorkingTreeOptions.  This simplifies all of the callers as they
no longer need to lookup core.filemode on their own.

A few locations were changed from always using a hardcoded "true"
on the file mode to passing what is actually configured in the
repository.  This is a behavior change, but corrects what should be
considered to be bugs as the core.filemode variable wasn't always
being used.

Change-Id: Idb176736fa0dc97af372f1d652a94ecc72fb457c
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>