The excellent extmerge plugin wants to tell the external merge tool about 4 files. Meld only likes to work with three files. Rather than patch meld, I found John's old suggestion on the mailing list and hacked it into a little meld wrapper that gives both bazaar and meld the interfaces they expect. If you are a kdiff3 user, well, extmerge works out of the box for you. I'm partial to meld though, so here is the glue you need (I put this in a script called meld-helper in ~/bin):
# Add this line to your ~/.bazaar/bazaar.conf # external_merge = 'meld-helper %r %b %t %o' rm $1 mv $3 $1 meld $2 $4 $1