Add completions to the dive location, buddy and divemaster entries

This way you can just type the first few characters of a location you've
been to before, and it will show you a list of possible completions.
Same for buddies and divemasters (which take the completions from a list
of people you've used before).

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>