graft: fix collision detection with origin revisions that are missing

When grafting something with a matching origin, it would normally be skipped:
  skipping already grafted revision 123 (23 also has origin 12)

But after stripping a graft origin, graft could fail with a reference to the
origin that no longer exists:
  abort: unknown revision '5c095ad7e90f871700f02dd1fa5012cb4498a2d4'!

Instead, detect that the origin is unknown and skip it anyway, like:
  skipping already grafted revision 8 (2 also has unknown origin 5c095ad7e90f871700f02dd1fa5012cb4498a2d4)

mercurial/commands.py

@@ -3183,14 +3183,23 b' def graft(ui, repo, *revs, **opts):'
         ctx = repo[rev]
         n = ctx.extra().get('source')
         if n in ids:
-            r = repo[n].rev()
+            try:
+                r = repo[n].rev()
+            except error.RepoLookupError:
+                r = None
             if r in revs:
                 ui.warn(_('skipping revision %s (already grafted to %s)\n')
                         % (r, rev))
                 revs.remove(r)
             elif ids[n] in revs:
-                ui.warn(_('skipping already grafted revision %s '
-                            '(%s also has origin %d)\n') % (ids[n], rev, r))
+                if r is None:
+                    ui.warn(_('skipping already grafted revision %s '
+                              '(%s also has unknown origin %s)\n')
+                            % (ids[n], rev, n))
+                else:
+                    ui.warn(_('skipping already grafted revision %s '
+                              '(%s also has origin %d)\n')
+                            % (ids[n], rev, r))
                 revs.remove(ids[n])
         elif ctx.hex() in ids:
             r = ids[ctx.hex()]

tests/test-graft.t

@@ -631,3 +631,15 b' graft works on complex revset'
   grafting revision 13
   grafting revision 19
   merging b
+
+
+Continue testing same origin policy, using revision numbers from test above
+but do some destructive editing of the repo:
+
+  $ hg up -qC 7
+  $ hg tag -l -r 13 tmp
+  $ hg --config extensions.mq= strip 2
+  saved backup bundle to $TESTTMP/a/.hg/strip-backup/5c095ad7e90f-backup.hg (glob)
+  $ hg graft tmp
+  skipping already grafted revision 8 (2 also has unknown origin 5c095ad7e90f871700f02dd1fa5012cb4498a2d4)
+  [255]