vifm: Add simple file and folder diff command

The newly added `:cmp` command can compare the contents of either two
files or two directories.

Works in two ways: If nothing is selected, compares the currently
hovered-over object with that in the other pane.
If things are selected, compares those instead.

terminal/.config/vifm/scripts/vifmdiff

@@ -0,0 +1,31 @@
+#!/bin/bash
+
+if [ $# != 2 ] ; then
+    echo 'Expected exactly two arguments'
+    exit 1
+fi
+
+if [ -f "$1" ] && [ -f "$2" ]; then
+    args=
+elif [ -d "$1" ] && [ -d "$2" ]; then
+    args='-r'
+else
+    if [ -f "$1" ]; then
+        type_of_1='file'
+    else
+        type_of_1='directory'
+    fi
+    if [ -f "$2" ]; then
+        type_of_2='file'
+    else
+        type_of_2='directory'
+    fi
+    echo "Arguments are of different type ($type_of_1/$type_of_2)"
+    exit 1
+fi
+
+if diff $args "$1" "$2" > /dev/null 2>&1; then
+    echo 'Equal'
+else
+    echo 'Not equal'
+fi