mingw.c: fix alternate screen using TERM=xterm-256color

With a recent MinGW64 update, the terminal does not switch between main
screen buffer and alternate screen buffer correctly when TERM is set to
"cygwin". In particular, after less and vim end, the command prompt is
reset to the top of the terminal window and old text remains on the
screen.

My tests show that TERM=xterm-256color works as intended.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>

Author: j6t

compat/mingw.c

@@ -2822,7 +2822,7 @@ static void setup_windows_environment(void)
 
 	/* simulate TERM to enable auto-color (see color.c) */
 	if (!getenv("TERM"))
-		setenv("TERM", "cygwin", 1);
+		setenv("TERM", "xterm-256color", 1);
 
 	/* calculate HOME if not set */
 	if (!getenv("HOME")) {