Correct mistake using kwargs
More tidying of release text
Some better reporting of things

1. In "muddle query checkout-branches", if a checkout VCS doesn't
   support branching, but has "no_follow" set, make that last
   evident.

2. When doing git pull/merge, if "git fetch" fails, we want to
   report the entire output, instead of just the error code
   (which is unhelpful at best). This is especially noticeable
   if we try to pull a branch that doesn't exist at the far end.