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
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273ce12ad8f1` will report some style violations on a very old commit).