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>

mergetools/bc3

@@ -16,5 +16,12 @@ merge_cmd () {
 }
 
 translate_merge_tool_path() {
-	echo bcompare
+	case $(uname -s) in
+	*MINGW*)
+		echo bcomp
+		;;
+	*)
+		echo bcompare
+		;;
+	esac
 }