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>