git merge with allow_bare option failed to reset workdir in the absorbed repo

[shijinglu]

If a submodule is opened during merge, we the absorbed repo have valid workree:

$ cat $(git meta root)/.git/modules/foo/bar/config
[core]
<...>
     workdir = ../../../foo/bar

However, this is not always true in git-meta. There are occasions workdir is set to null even though it should not be. aka:

$ cat $(git meta root)/.git/modules/foo/bar/config
[core]
<...>
     workdir = null

The reasons is that, if the ALLOW_BARE option is enabled for normal merge, git meta will first try to merge the submodule inside the absorbed repo and treat it as a bare repo. And if there are any merge conflicts, git meta will then re open the subrepo. If a subrepo was first created as the bare repo, and then opened fully, git-meta won't reset the workdir and this leads to an empty submodule under foo/bar.