Commit.exports(): use #join (to work in Ruby 1.9)

Commit.exports(): use #join (to work in Ruby 1.9)

When rcs-fast-export.rb was run in "directory" mode under Ruby 1.9, then
log messages that were output came out in "array" format rather than plain
text, as in
  ["log line 1\n","log line 2\n"]

Switching from ".to_s" to ".join" allows this to work in both Ruby 1.8 and
1.9.

(This is fix is similar to commit 3dbc2f0ee9e36444c6927ad1ec7dc163f08d6f2f
from Jan 12 2011, which makes a similar change to the log messages output
when the script processes a single RCS file.)