Merge branch 'ef/mingw-tty-getpass'

Update getpass() emulation for MinGW.

* ef/mingw-tty-getpass:
  mingw: get rid of getpass implementation
  mingw: reuse tty-version of git_terminal_prompt
  compat/terminal: separate input and output handles
  compat/terminal: factor out echo-disabling
  mingw: make fgetc raise SIGINT if apropriate
  mingw: correct exit-code for SIGALRM's SIG_DFL