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.)