git-svn, perl/Git.pm: extend and use Git->prompt method for querying users

git-svn, perl/Git.pm: extend and use Git->prompt method for querying users

git-svn reads usernames and other user queries from an interactive
terminal. This cause GUIs (w/o STDIN connected) to hang waiting forever
for git-svn to complete (http://code.google.com/p/tortoisegit/issues/detail?id=967).

This change extends the Git::prompt helper, so that it can also be used
for non password queries, and makes use of it instead of using
hand-rolled prompt-response code that only works with the interactive
terminal.

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>