git-gui: Honor core.excludesfile when listing extra files

git-gui: Honor core.excludesfile when listing extra files

Recent git versions have a git-status that honors the core.excludesfile
configuration option when it reports on untracked files.  Unfortunately
I missed the introduction of this configuration option in the core
porcelain implementation, so it was not reflected here in git-gui.

Found and reported by Lars Noschinski <lars@public.noschinski.de>.

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