git-svn, perl/Git.pm: add central method for prompting passwords

git-svn, perl/Git.pm: add central method for prompting passwords

git-svn reads passwords from an interactive terminal or by using
GIT_ASKPASS helper tool. 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).

Commit 56a853b62c0ae7ebaad0a7a0a704f5ef561eb795 also tried to solve
this issue, but was incomplete as described above.

Instead of using hand-rolled prompt-response code that only works with the
interactive terminal, a reusable prompt() method is introduced in this commit.

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