[bug] Multiline value should error if the next line is section

[Shulhan]

## Environment

OS: Arch Linux
Git version: git@next d54016d9e

## Reproduction Steps

(1) Create the following `git.config`,

```
[alias]
        tree = --no-pager log --graph \
                --date=format:'%Y-%m-%d' \
                --pretty=format:'%C(auto,dim)%ad %<(7,trunc) %an %Creset%m %h 
%s %Cgreen%d%Creset' \
                --exclude="*/production" \
                --exclude="*/dev-*" \
                -n 20 \
[user]
        name = Shulhan
        email = m...@kilabit.info
```

(2) Run `git config -f git.config -l`


## Expected Result

Error message,

  fatal: bad config line 9 at git.config


## Actual Result

The command print the following output,

```
alias.tree=--no-pager log --graph
--date=format:'%Y-%m-%d'
--pretty=format:'%C(auto,dim)%ad %<(7,trunc) %an %Creset%m %h %s 
%Cgreen%d%Creset' --exclude=*/production                 --exclude=*/dev-*      
           -n 20 [user]
alias.name=Shulhan
alias.email=m...@kilabit.info
```