revbranchcache: return uncached branchinfo for nullrev (issue4683)

This fixes the crash caused by "branch(null)" revset. No cache should be
necessary for nullrev because changelog.branchinfo(nullrev) does not involve
IO operation.

Note that the problem of "branch(wdir())" isn't addressed by this patch.
"wdir()" will raise TypeError in many places because of None. This is the
reason why "wdir()" is still experimental.

mercurial/branchmap.py

@@ -341,6 +341,10 b' class revbranchcache(object):'
         changelog = self._repo.changelog
         rbcrevidx = rev * _rbcrecsize
 
+        # avoid negative index, changelog.read(nullrev) is fast without cache
+        if rev == nullrev:
+            return changelog.branchinfo(rev)
+
         # if requested rev is missing, add and populate all missing revs
         if len(self._rbcrevs) < rbcrevidx + _rbcrecsize:
             self._rbcrevs.extend('\0' * (len(changelog) * _rbcrecsize -

tests/test-revset.t

@@ -1602,6 +1602,14 b' prepare repository that has "default" br'
   $ echo default5 >> a
   $ hg ci -m5
 
+"null" revision belongs to "default" branch (issue4683)
+
+  $ log 'branch(null)'
+  0
+  1
+  4
+  5
+
 "null" revision belongs to "default" branch, but it shouldn't appear in set
 unless explicitly specified (issue4682)