convert: only consider shamap revisions converted if they still exists

Rollback or strip could leave a Mercurial repo with a shamap with revisions no
longer in the repository.

To ensure reliable conversions we now check that the commit actually exists and
consider it non-existing if it doesn't exist.

hgext/convert/convcmd.py

@@ -173,8 +173,12 b' class converter(object):'
         parents = {}
         while visit:
             n = visit.pop(0)
-            if n in known or n in self.map:
+            if n in known:
                 continue
+            if n in self.map:
+                m = self.map[n]
+                if m == SKIPREV or self.dest.hascommitfrommap(m):
+                    continue
             known.add(n)
             self.ui.progress(_('scanning'), len(known), unit=_('revisions'))
             commit = self.cachecommit(n)

tests/test-convert-hg-sink.t

@@ -526,8 +526,11 b' Conversion after rollback'
   scanning source...
   sorting...
   converting...
+  0 extra f+a-only change
 
   $ hg -R a log -G -T '{rev} {desc|firstline} ({files})\n'
+  o  3 extra f+a-only change (f)
+  |
   o  2 x (f)
   |
   @  1 a (f)