git svn dcommit: new option --interactive.

git svn dcommit: new option --interactive.

Allow the user to check the patch set before it is commited to SVN. It is
then possible to accept/discard one patch, accept all, or quit.

This interactive mode is similar with 'git send email' behaviour. However,
'git svn dcommit' returns as soon as one patch is discarded.
Part of the code was taken from git-send-email.perl (see 'ask' function)

Tests several combinations of potential answers to
'git svn dcommit --interactive'. For each of them, test whether patches
were commited to SVN or not.

Thanks-to Eric Wong <normalperson@yhbt.net> for the initial idea.

Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Frédéric Heitzmann <frederic.heitzmann@gmail.com>