revert: small refactoring in the way backup value are handled

The current backup value may have two different values:

  1. Do not try to do backup
  2. Do backup if applicable

We are about to move to:

  1. Do not try to do backup
  2. Do backup if applicable
  3. Do backup in all cases

So we change the current values to make room for the new one.

mercurial/cmdutil.py

@@ -2626,10 +2626,13 def revert(ui, repo, ctx, parents, *pats
                    'unknown': (None, _('file not managed: %s\n')),
                   }
 
-
-        # should we do a backup?
-        backup = not opts.get('no_backup')
-        discard = False
+        # "constant" that convey the backup strategy.
+        # All set to `discard` if `no-backup` is set do avoid checking
+        # no_backup lower in the code.
+        backup = 2  # unconditionally do backup
+        discard = 0 # never do backup
+        if opts.get('no_backup'):
+            backup = discard
 
         disptable = (
             # dispatch table: