mergetools: use the correct tool for Beyond Compare 3 on Windows

On Windows the bcompare tool launches a graphical program and does
not wait for it to terminate. A separate 'bcomp' tool is provided which
will wait for the view to exit so we use this instead.

Reported-by: Werner BEROUX <werner@beroux.com>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

mergetools/bc3

@@ -16,5 +16,10 @@ merge_cmd () {
 }
 
 translate_merge_tool_path() {
-	echo bcompare
+	if type bcomp >/dev/null 2>/dev/null
+	then
+		echo bcomp
+	else
+		echo bcompare
+	fi
 }