`magit-diff-with-mark': better prompt and completion

When prompting for a rev to diff the currently marked commit with,
report the latter, and default to the currently checked out branch
unless that *is* the marked commit.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #914 from praet/diff-with-mark-v2

diff-with-mark v2