fbamend: abort if preamend bookmark points to the current commit

Differential Revision: https://phabricator.intern.facebook.com/D3436696

fbamend.py

@@ -305,11 +305,17 @@ def fixupamend(ui, repo):
             raise error.Abort(_('no bookmark %s') % preamendname,
                              hint=_('check if your bookmark is active'))
 
-        ui.status(_("rebasing the children of %s\n") % (preamendname))
-
         old = repo[preamendname]
+        if old == current:
+            hint = _('please examine smartlog and rebase your commits manually')
+            err = _('cannot automatically determine what to rebase '
+                    'because bookmark "%s" points to the current commit') % \
+                   preamendname
+            raise error.Abort(err, hint=hint)
         oldbookmarks = old.bookmarks()
 
+        ui.status(_("rebasing the children of %s\n") % (preamendname))
+
         active = bmactive(repo)
         opts = {
             'rev' : [str(c.rev()) for c in old.descendants()],

tests/test-fbamend.t

@@ -651,4 +651,21 @@ Check whether unamend works with dirty working directory
   A c
   A d
   R b
+  $ cd ..
+
+Check whether fbamend warns people if preamend bookmark is the same as an active
+bookmark
+  $ hg init messedpreamed && cd messedpreamed
+  $ echo root > root && hg ci -Am root
+  adding root
+  $ hg book -r . activebook
+  $ hg book -r . activebook.preamend
+  $ hg up activebook
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (activating bookmark activebook)
+  $ hg amend --fixup
+  abort: cannot automatically determine what to rebase because bookmark "activebook.preamend" points to the current commit
+  (please examine smartlog and rebase your commits manually)
+  [255]
+