[Rd] infinite loop in abbreviate() (PR#4564)

[tlumley at u.washington.edu]

abbreviate() will go into an infinite loop if two elements of the argument
differ only by leading space

eg
  abbreviate(c("A"," A"), 4)

This is in R 1.8.0 (both OS X and Linux).

The problem appears to be that duplicates are removed before leading and
trailing spaces.

	-thomas