git-svn: print and flush authentication prompts to STDERR
git-svn: print and flush authentication prompts to STDERR

People that redirect STDOUT output should always see STDERR
prompts interactively.

STDERR should always be flushed without buffering, so
they should always show up.  If that is unset, we still
explicitly flush by calling STDERR->flush.

The svn command-line client prompts to STDERR, too.

Signed-off-by: Eric Wong <8c2951295f8292b0c03727bc4652981172298eae@yhbt.net>
Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git