Add bitrot detection to "diff" command

This introduces a new modifier to the output of the diff command. It
appears whenever two files being compared only differ in their content
but not in their metadata. As far as we know, under normal
circumstances, this should only ever happen if some kind of bitrot has
happened in the source file. The prerequisite for this detection to work
is that the right-side snapshot of the comparison has been created with
"backup --force".

cmd/restic/cmd_diff.go

@@ -27,6 +27,7 @@ directory:
 * U  The metadata (access mode, timestamps, ...) for the item was updated
 * M  The file's content was modified
 * T  The type was changed, e.g. a file was made a symlink
+* ?  Bitrot detected: The file's content has changed but all metadata is the same
 
 To only compare files in specific subfolders, you can use the
 "<snapshotID>:<subfolder>" syntax, where "subfolder" is a path within the
@@ -272,6 +273,14 @@ func (c *Comparer) diffTree(ctx context.Context, stats *DiffStatsContainer, pref
 				!reflect.DeepEqual(node1.Content, node2.Content) {
 				mod += "M"
 				stats.ChangedFiles++
+
+				node1NilContent := node1
+				node2NilContent := node2
+				node1NilContent.Content = nil
+				node2NilContent.Content = nil
+				if node1NilContent.Equals(*node2NilContent) {
+					mod += "?"
+				}
 			} else if c.opts.ShowMetadata && !node1.Equals(*node2) {
 				mod += "U"
 			}