git-gui: Reworded 'Include' to 'Add' to match core Git.

Now that git-add is a first class citizen in core Git (Nico's 366bfcb6)
users may start to expect the term 'add' to refer to the act of including
a file's changes into a commit.  So I'm replacing all uses of the term
'Include' in the UI with 'Add'.

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