join: improve performance when operating on whole lines

join: improve performance when operating on whole lines

Following on from commit f86bb696, 01-02-2010,
"join: make -t '' operate on the whole line".
Bypassing the delimiter search in this case,
gives about an 8% performance boost.

* src/join (xfields): Don't bother looking for '\n'
in the data, which we know won't be present.