git: initialize `extra` to have at least the branch name for nullid

Otherwise, this crashes trying to convert to local encoding:

    ...
      File "/mnt/c/Users/Matt/hg/mercurial/logcmdutil.py", line 333, in _show
        branch = ctx.branch()
      File "/mnt/c/Users/Matt/hg/mercurial/context.py", line 675, in branch
        return encoding.tolocal(self._changeset.extra.get(b"branch"))
      File "/mnt/c/Users/Matt/hg/mercurial/encoding.py", line 181, in tolocal
        if isasciistr(s):
    TypeError: a bytes-like object is required, not 'NoneType'

This was originally reported to the thg bug tracker.

https://foss.heptapod.net/mercurial/tortoisehg/thg/-/issues/5629

Differential Revision: https://phab.mercurial-scm.org/D10528

hgext/git/gitlog.py

@@ -221,9 +221,10 b' class changelog(baselog):'
             n = self.node(nodeorrev)
         else:
             n = nodeorrev
+        extra = {b'branch': b'default'}
         # handle looking up nullid
         if n == nullid:
-            return hgchangelog._changelogrevision(extra={}, manifest=nullid)
+            return hgchangelog._changelogrevision(extra=extra, manifest=nullid)
         hn = gitutil.togitnode(n)
         # We've got a real commit!
         files = [
@@ -253,7 +254,7 b' class changelog(baselog):'
             filesremoved=filesremoved,
             description=c.message.encode('utf8'),
             # TODO do we want to handle extra? how?
-            extra={b'branch': b'default'},
+            extra=extra,
         )
 
     def ancestors(self, revs, stoprev=0, inclusive=False):

tests/test-git-interop.t

@@ -57,6 +57,12 b' Test some edge cases around a commitless'
   $ hg init --git
   $ hg heads
   [1]
+  $ hg tip
+  changeset:   -1:000000000000
+  tag:         tip
+  user:        
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  
   $ cd ..
 
 Make a new repo with git: