mingw: $env:TERM="xterm-256color" for newer OSes

For Windows builds >= 15063 set $env:TERM to "xterm-256color" instead of
"cygwin" because they have a more capable console system that supports
this. Also set $env:COLORTERM="truecolor" if unset.

$env:TERM is initialized so that ANSI colors in color.c work, see
29a3963 (Win32: patch Windows environment on startup, 2012-01-15).

See #3629 regarding problems caused by always setting
$env:TERM="cygwin".

This is the same heuristic used by the Cygwin runtime.

Signed-off-by: Rafael Kitover <rkitover@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mingw.c

@@ -2642,9 +2642,20 @@ static void setup_windows_environment(void)
 		convert_slashes(tmp);
 	}
 
-	/* simulate TERM to enable auto-color (see color.c) */
-	if (!getenv("TERM"))
-		setenv("TERM", "cygwin", 1);
+
+	/*
+	 * Make sure TERM is set up correctly to enable auto-color
+	 * (see color.c .) Use "cygwin" for older OS releases which
+	 * works correctly with MSYS2 utilities on older consoles.
+	 */
+	if (!getenv("TERM")) {
+		if ((GetVersion() >> 16) < 15063)
+			setenv("TERM", "cygwin", 0);
+		else {
+			setenv("TERM", "xterm-256color", 0);
+			setenv("COLORTERM", "truecolor", 0);
+		}
+	}
 
 	/* calculate HOME if not set */
 	if (!getenv("HOME")) {