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".

changelog/unreleased/pull-4526

@@ -0,0 +1,11 @@
+Enhancement: Add bitrot detection to `diff` command
+
+The output of the `diff` command now includes the modifier `?` for files
+to indicate bitrot in backed up files. It will appear whenever there is a
+difference in content while the metadata is exactly the same. Since files with
+unchanged metadata are normally not read again when creating a backup, the
+detection is only effective if the right-hand side of the diff has been created
+with "backup --force".
+
+https://github.com/restic/restic/issues/805
+https://github.com/restic/restic/pull/4526