revert: distinguish between "check" and "backup" strategy

"check" behaves as backup did before. We check if the current file differs
from destination and we create a backup if it does. This is used for untracked
files that will be overwritten by formerly-deleted files. We have to do the manual
check since no status output can provide the content comparison.

"backup" is now doing unconditional backup. This can be used for files seen as
modified compared to both the target and the working directory. In such a case, we
know that the file differs from target without actually comparing any content.

This new "backup" strategy will be especially useful in the case of files added
between the target and the working directory -parent- with additional modifications
in the working directory -itself-. In that case we know we need to back it up, but we
cannot run the content check as the files does not exists in target.

mercurial/cmdutil.py

@@ -2629,10 +2629,12 b' def revert(ui, repo, ctx, parents, *pats'
         # "constant" that convey the backup strategy.
         # All set to `discard` if `no-backup` is set do avoid checking
         # no_backup lower in the code.
+        # These values are ordered for comparison purposes
         backup = 2  # unconditionally do backup
+        check = 1   # check if the existing file differs from target
         discard = 0 # never do backup
         if opts.get('no_backup'):
-            backup = discard
+            backup = check = discard
 
         disptable = (
             # dispatch table:
@@ -2656,11 +2658,11 b' def revert(ui, repo, ctx, parents, *pats'
             # Removed since  target, before working copy parent
             (removed,       actions['add'],      discard),
             # Same as `removed` but an unknown file exists at the same path
-            (removunk,      actions['add'],      backup),
+            (removunk,      actions['add'],      check),
             # Removed since targe, marked as such in working copy parent
             (dsremoved,     actions['undelete'], discard),
             # Same as `dsremoved` but an unknown file exists at the same path
-            (dsremovunk,    actions['undelete'], backup),
+            (dsremovunk,    actions['undelete'], check),
             ## the following sets does not result in any file changes
             # File with no modification
             (clean,         actions['noop'],     discard),
@@ -2683,12 +2685,13 b' def revert(ui, repo, ctx, parents, *pats'
                     continue
                 if xlist is not None:
                     xlist.append(abs)
-                    if (dobackup and wctx[abs].cmp(ctx[abs])):
-                        bakname = "%s.orig" % rel
-                        ui.note(_('saving current version of %s as %s\n') %
-                                (rel, bakname))
-                        if not opts.get('dry_run'):
-                            util.rename(target, bakname)
+                    if dobackup and (backup <= dobackup
+                                     or wctx[abs].cmp(ctx[abs])):
+                            bakname = "%s.orig" % rel
+                            ui.note(_('saving current version of %s as %s\n') %
+                                    (rel, bakname))
+                            if not opts.get('dry_run'):
+                                util.rename(target, bakname)
                     if ui.verbose or not exact:
                         if not isinstance(msg, basestring):
                             msg = msg(abs)