Without /dev/tty support, git_terminal_prompt simply ignores the
'echo'-parameter. On Windows we can do better by clevering up our
getpass-implementation a bit so it can conditionally echo.

While we're at it, plug a small memory-leak by returning a pointer
to a static strbuf instead of detaching it. This is the same thing
the /dev/tty-version of git_terminal_prompt does, and the callee
doesn't expect to have to free it's memory.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>