git-svn: move Git::SVN::Prompt into its own file

git-svn: move Git::SVN::Prompt into its own file

git-svn.perl is very long (around 6500 lines) and although it is
nicely split into modules, some new readers do not even notice --- it
is too distracting to see all this functionality collected in a single
file.

Splitting it into multiple files would make it easier for people
to read individual modules straight through and to experiment with
components separately.

Let's start with Git::SVN::Prompt.  For simplicity, we install this as
a module in the standard search path, just like the existing Git and
Git::I18N modules.  In the process, add a manpage explaining its
interface and that it is not likely to be useful for other projects to
avoid confusion.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Eric Wong <normalperson@yhbt.net>