hgweb: don't raise an exception when displying empty repos The nullid node claims it's in the default branch, but the branch dict is empty. This fixes the main symptom from issue696, but we may want to set branchtags()['default'] = nullid somewhere for empty repos.

adding a
adding b
copy .hg/patches to .hg/patches.1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
M b
a
b
merging with queue at: .hg/patches.1
applying rm_a
Now at: rm_a
b
Patch queue now empty