merge branch with updated submodule leaves repo in modified state

[Daniel Bosk <dbosk@xxxxxx>]

Hi all!

`git merge`: if a submodule is updated in the merged branch, the new 
version is not checked out after the merge. Is this correct behaviour?

`git switch` correctly updates the submodules when switching between the 
branches. I would expect `git merge` to leave the things in the most 
up-to-date state.

I tested this with the following settings.

  $ git version
  git version 2.24.0

  $ git config -l
  push.default=matching
  diff.submodule=log
  diff.colorwords=true
  diff.algorithm=minimal
  diff.exit-code=true
  commit.gpgsign=true
  submodule.fetchjobs=0
  submodule.recurse=true
  log.follow=true
   
MWE:

```
mkdir repoA repoB

# init repos with READMEs
cd repoA
git init
echo "A" > README
git add README
git commit -m "Adds repoA README" README

cd ../repoB
git init
echo "B" > README
git add README
git commit -m "Adds repoB README" README

# construct nested structure: repoA/repoB
cd ../repoA
git submodule add ../repoB/.git
git commit -m "Adds repoB submodule"

# update repoB in branch
git switch -c update-repoB-submodule
cd repoB
echo "B updated" > README
git commit -m "Updates repoB README" README
cd ..
git commit -m "Updates repoB submodule" repoB
```

Now, doing `git switch master` will have the correct version of repoB, 
where `cat repoB/README` outputs "B". A `git switch update-repoB-submodule`
from there will yield `cat repoB/README` output "B updated".

However, switching to master and then merging will leave repoB at the 
old commit:
```
git switch master
git merge update-repoB-submodule
```
A `git status` says there are new commits in repoB, it's actually a 
rewind which excludes the new commit (changing `repoB/README`), so `cat 
repoB/README` outputs "B" instead of "B updated". It feels unintended to 
get a repo which is in a modified state after a clean merge. Bug?

Cheers,
Daniel

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