`magit-diff-working-tree': make prompt more informative

Signed-off-by: Pieter Praet <pieter@praet.org>
`magit-checkout': make prompt more informative

Report current branch in prompt, so we know why frantically
hitting TAB doesn't help with completing a branch name.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #750 from praet/prompts

Better Prompts.