fetchbzr, fetchdarcs, fetchhg: use rev attr

This makes it match the behaviour of fetchgit and fetchsvn, so it's
easier to write scripts that support all of them.

pkgs/build-support/fetchbzr/builder.sh

@@ -1,9 +1,9 @@
 source "$stdenv/setup"
 
-header "exporting \`$url' (revision $revision) into \`$out'"
+header "exporting \`$url' (revision $rev) into \`$out'"
 
 # Perform a lightweight checkout so that we don't end up importing
 # all the repository's history.
-bzr -Ossl.cert_reqs=none export -r "$revision" --format=dir "$out" "$url"
+bzr -Ossl.cert_reqs=none export -r "$rev" --format=dir "$out" "$url"
 
 stopNest