Combine message of consecutive squashed commits in one go

Given a git log like this:
```
1 add feature
2 fix something
3 fix tidy
```
Currently, git-revise only combines messages of squashed commit one by one.
Which means that it combines 1 and 2 saved, then combine the result to 3.
git-rebase combines 1, 2, 3 in one step.
