merge: don't overwrite file untracked after remove, abort with 'untracked files'

Merge could overwrite untracked files and cause data loss.

Instead we now handle the 'local side removed file and has untracked file
instead' case as the 'other side added file that local has untracked' case:

  FILE: untracked file exists
  abort: untracked files in working directory differ from files in requested revision

It could perhaps make sense to create .orig files when overwriting, either
instead of aborting or when overwriting anyway because of force ... but for now
we stay consistent with similar cases.

mercurial/merge.py

@@ -343,7 +343,12 b' def manifestmerge(repo, wctx, p2, pa, br'
                 else:
                     actions.append((f, "g", (fl2,), "remote created"))
         elif n2 and n2 != ma[f]:
-            prompts.append((f, "dc")) # prompt deleted/changed
+            different = _checkunknownfile(repo, wctx, p2, f)
+            if not force and different:
+                aborts.append((f, "ud"))
+            else:
+                # if different: old untracked f may be overwritten and lost
+                prompts.append((f, "dc")) # prompt deleted/changed
 
     for f, m in sorted(aborts):
         if m == "ud":

tests/test-merge-remove.t

@@ -85,3 +85,30 b' Reverting foo1 and bar:'
 
   $ hg diff
 
+Merge should not overwrite local file that is untracked after remove
+
+  $ rm *
+  $ hg up -qC
+  $ hg rm bar
+  $ hg ci -m 'remove bar'
+  $ echo 'memories of buried pirate treasure' > bar
+  $ hg merge
+  bar: untracked file differs
+  abort: untracked files in working directory differ from files in requested revision
+  [255]
+  $ cat bar
+  memories of buried pirate treasure
+
+Those who use force will lose
+
+  $ hg merge -f
+  remote changed bar which local deleted
+  use (c)hanged version or leave (d)eleted? c
+  merging foo1 and foo to foo1
+  1 files updated, 1 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ cat bar
+  bleh
+  $ hg st
+  M bar
+  M foo1