convert: return commit objects for revisions in the revmap

Source revision data that exists in the revmap are ignored when pulling
data from Perforce as we consider them already imported. In case where
the `convertcmd.convert` algorithm requests a commit object for such
a revision we are creating it.  This is usually the case for parent of
the first imported revision.

hgext/convert/p4.py

@@ -321,7 +321,13 b' class p4_source(common.converter_source)'
         return marshal.load(stdout)
 
     def getcommit(self, rev):
-        return self.changeset[rev]
+        if rev in self.changeset:
+            return self.changeset[rev]
+        elif rev in self.revmap:
+            d = self._fetch_revision(rev)
+            return self._construct_commit(d, parents=None)
+        raise error.Abort(
+            _("cannot find %s in the revmap or parsed changesets") % rev)
 
     def gettags(self):
         return {}