Cygwin: console: Handle Unicode surrogate pairs.

When running Cygwin's Bash in the Windows Terminal (see
https://docs.microsoft.com/en-us/windows/terminal/ for details), Cygwin
is receiving keyboard input in the form of UTF-16 characters.

UTF-16 has that awkward challenge that it cannot map the full Unicode
range, and to make up for it, there are the ranges U+D800-U+DBFF and
U+DC00-U+DFFF which are illegal except when they come in a pair encoding
for Unicode characters beyond U+FFFF.

Cygwin does not handle such surrogate pairs correctly at the moment, as
can be seen e.g. when running Cygwin's Bash in the Windows Terminal and
then inserting an emoji (e.g. via Windows + <dot>, which opens an emoji
picker on recent Windows versions): Instead of showing an emoji, this
shows the infamous question mark in a black triangle, i.e. the invalid
Unicode character.

Let's special-case surrogate pairs in this scenario.

This fixes https://github.com/git-for-windows/git/issues/3281

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

winsup/cygwin/fhandler_console.cc

@@ -918,8 +918,23 @@ fhandler_console::process_input_message (void)
 	      nread = strlen (toadd);
 	    }
 	  else
+	    {
+	      WCHAR second = unicode_char >= 0xd800 && unicode_char <= 0xdbff
+		  && i + 1 < total_read ?
+		  input_rec[i + 1].Event.KeyEvent.uChar.UnicodeChar : 0;
+
+	      if (second < 0xdc00 || second > 0xdfff)
 		{
 		  nread = con.con_to_str (tmp + 1, 59, unicode_char);
+		}
+	      else
+		{
+		  /* handle surrogate pairs */
+		  WCHAR pair[2] = { unicode_char, second };
+		  nread = sys_wcstombs (tmp + 1, 59, pair, 2);
+		  i++;
+		}
+
 	      /* Determine if the keystroke is modified by META.  The tricky
 		 part is to distinguish whether the right Alt key should be
 		 recognized as Alt, or as AltGr. */

winsup/cygwin/release/3.3.3

@@ -16,3 +16,9 @@ Bug Fixes
 - Fix long-standing problem that new files don't get created with the
   FILE_ATTRIBUTE_ARCHIVE DOS attribute set.
   Addresses: https://cygwin.com/pipermail/cygwin/2021-November/249909.html
+
+- Handle Unicode surrogate pairs in console. Cygwin console does not
+  handle surrogate pairs correctly at the moment.  Fix issue that
+  running bash in Windows Terminal and inserting an emoji does not
+  work as expected.
+  Addresses: https://github.com/git-for-windows/git/issues/3281