git-gui: Change more 'include' language to 'add'.

I just found a whole slew of places where we still were using the term
'include' rather than 'add' to refer to the act of updating the index
with modifications from the working directory.  To be consistent with
all Git documentation and command line tools, these should be 'add'.

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