gitlog: add tiprev() function

Lots of stuff was broken because this was missing.

Differential Revision: https://phab.mercurial-scm.org/D9402

hgext/git/gitlog.py

@@ -148,6 +148,14 b' class changelog(baselog):'
         )
         return (int(r[0]) for r in t)
 
+    def tiprev(self):
+        t = self._db.execute(
+            'SELECT rev FROM changelog '
+            'ORDER BY REV DESC '
+            'LIMIT 1'
+        )
+        return next(t)
+
     def _partialmatch(self, id):
         if nodemod.wdirhex.startswith(id):
             raise error.WdirUnsupported