revert: properly back up added files with local modification

These files were previously not backed up because the backup mechanism was not
smart enough. This leads to data lose for the user since uncommitted contents
were discarded.

We now properly move the modified version to <filename>.orig before deleting it.

We have to use a small hack to do a different action if "--no-backup" is
specified. This is needed because the backup process is actually a move (not a
copy) so the file is already missing when we backup. The internet kitten is a
bit disapointed about that, but such is life.

This patch concludes the "lets refactor revert" phases. We can now open the
"Lets find stupid bug with renames and merge" phases.

I'm sure that now that the code is clearer we could do it in another simpler
way, but I consider the current improvement good enough for now.

mercurial/cmdutil.py

@@ -2640,6 +2640,10 b' def revert(ui, repo, ctx, parents, *pats'
         if opts.get('no_backup'):
             backup = check = discard
 
+        backupanddel = actions['remove']
+        if not opts.get('no_backup'):
+            backupanddel = actions['drop']
+
         disptable = (
             # dispatch table:
             #   file state
@@ -2658,7 +2662,7 b' def revert(ui, repo, ctx, parents, *pats'
             # Added in working directory
             (dsadded,       actions['forget'],   discard),
             # Added since target, have local modification
-            (modadded,      actions['remove'],   discard),
+            (modadded,      backupanddel,        backup),
             # Added since target but file is missing in working directory
             (deladded,      actions['drop'],   discard),
             # Removed since  target, before working copy parent

tests/test-revert.t

@@ -940,16 +940,12 b' Compare resulting directory with revert '
 The diff is filtered to include change only. The only difference should be
 additional `.orig` backup file when applicable.
 
-Misbehavior:
-
-- no backup for
-| - added_wc (DATA LOSS)
-
   $ python ../dircontent.py > ../content-base-all.txt
   $ cd ..
   $ diff -U 0 -- content-base.txt content-base-all.txt | grep _
   +parent added_untracked-clean
   +wc     added_untracked-wc
+  +wc     added_wc.orig
   +wc     clean_untracked-wc.orig
   +wc     clean_wc.orig
   +wc     missing_untracked-wc