cmdutil: minor refactoring of changeset_printer._show
- use ctx.branch() instead of directly accessing the extra field "branch"
- move definitions of locals ('extra' and 'branch') down to where they
are used
#!/bin/sh
# http://mercurial.selenic.com/bts/issue433
hg init a
cd a
echo a > a
hg commit -Ama
hg parents -r 0 doesnotexist
true