Merge branch 'ss/svn-prompt' into maint

The way "git svn" asked for password using SSH_ASKPASS and
GIT_ASKPASS was not in line with the rest of the system.

* ss/svn-prompt:
  git-svn, perl/Git.pm: extend and use Git->prompt method for querying users
  perl/Git.pm: Honor SSH_ASKPASS as fallback if GIT_ASKPASS is not set
  git-svn, perl/Git.pm: add central method for prompting passwords