merge: before cd/dc prompt, check that changed side really changed

Before, merging would in some cases ask "wrong" questions about
"changed/deleted" conflicts ... and even do it before the resolve phase where
they can be postponed, re"resolved" or answered in bulk operations.

Instead, check that the content of the changed file really did change.

Reading and comparing file content is expensive and should be avoided before
the resolve phase. Prompting the user is however even more expensive. Checking
the content here is thus better.

The 'f in ancestors[0]' should not be necessary but is included to be extra
safe.

mercurial/merge.py

@@ -827,7 +827,10 b' def calculateupdates(repo, wctx, mctx, a'
 
     # Prompt and create actions. TODO: Move this towards resolve phase.
     for f, args, msg in actions['cd']:
-        if repo.ui.promptchoice(
+        if f in ancestors[0] and not wctx[f].cmp(ancestors[0][f]):
+            # local did change but ended up with same content
+            actions['r'].append((f, None, "prompt same"))
+        elif repo.ui.promptchoice(
             _("local changed %s which remote deleted\n"
               "use (c)hanged version or (d)elete?"
               "$$ &Changed $$ &Delete") % f, 0):
@@ -838,7 +841,10 b' def calculateupdates(repo, wctx, mctx, a'
 
     for f, args, msg in actions['dc']:
         flags, = args
-        if repo.ui.promptchoice(
+        if f in ancestors[0] and not mctx[f].cmp(ancestors[0][f]):
+            # remote did change but ended up with same content
+            pass # don't get = keep local deleted
+        elif repo.ui.promptchoice(
             _("remote changed %s which local deleted\n"
               "use (c)hanged version or leave (d)eleted?"
               "$$ &Changed $$ &Deleted") % f, 0) == 0:

tests/test-issue3084.t

@@ -262,8 +262,6 b' Ancestor: normal Parent: normal-same P'
 
   $ hg up -Cqr normal-same
   $ hg merge -r large
-  local changed f which remote deleted
-  use (c)hanged version or (d)elete? c
   getting changed largefiles
   1 largefiles updated, 0 removed
   1 files updated, 0 files merged, 1 files removed, 0 files unresolved
@@ -275,11 +273,9 b' swap'
 
   $ hg up -Cqr large
   $ hg merge -r normal-same
-  remote changed f which local deleted
-  use (c)hanged version or leave (d)eleted? c
   getting changed largefiles
-  1 largefiles updated, 0 removed
-  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  0 largefiles updated, 0 removed
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
   (branch merge, don't forget to commit)
   $ cat f
   large
@@ -387,8 +383,6 b' Ancestor: large Parent: large-same P'
 
   $ hg up -Cqr large-same
   $ hg merge -r normal
-  local changed .hglf/f which remote deleted
-  use (c)hanged version or (d)elete? c
   getting changed largefiles
   0 largefiles updated, 0 removed
   1 files updated, 0 files merged, 1 files removed, 0 files unresolved
@@ -400,11 +394,7 b' swap'
 
   $ hg up -Cqr normal
   $ hg merge -r large-same
-  remote changed .hglf/f which local deleted
-  use (c)hanged version or leave (d)eleted? c
-  getting changed largefiles
-  0 largefiles updated, 0 removed
-  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
   (branch merge, don't forget to commit)
   $ cat f
   normal