Cygwin: pty: Prevent garbage remained in read ahead buffer.

- After commit 29431fcb5b, the issue
  reported in https://cygwin.com/pipermail/cygwin/2020-May/245057.html
  occurs. This is caused by the following mechanism. Cygwin less
  called from non-cygwin git is executed under /dev/cons* rather
  than /dev/pty* because parent git process only inherits pseudo
  console handle. Therefore, less sets ICANON flag for /dev/cons*
  rather than original /dev/pty*. When pty is switched to non-cygwin
  git process, line_edit() is used in fhandler_pty_master::write()
  only to set input_available_event and read ahead buffer is supposed
  to be flushed in accept_input(). However, ICANON flag is not set
  for /dev/pty*, so accept_input() is not called unless newline
  is entered. As a result, the input data remains in the read ahead
  buffer. This patch fixes the issue.

winsup/cygwin/fhandler.h

@@ -328,7 +328,7 @@ class fhandler_base
 
   virtual bool get_readahead_valid () { return raixget () < ralen (); }
   int puts_readahead (const char *s, size_t len = (size_t) -1);
-  int put_readahead (char value);
+  virtual int put_readahead (char value);
 
   int get_readahead ();
   int peek_readahead (int queryput = 0);
@@ -2381,6 +2381,7 @@ public:
   int process_slave_output (char *buf, size_t len, int pktmode_on);
   void doecho (const void *str, DWORD len);
   int accept_input ();
+  int put_readahead (char value);
   int open (int flags, mode_t mode = 0);
   void open_setup (int flags);
   ssize_t __stdcall write (const void *ptr, size_t len);