commit on the last commit of of the set corrupts state

If you commit the last commit of the merge, the merge isn't marked finished and `continue` does:
```
$ mergify continue
cat: .git/MERGIFY_CURRENT_HASH: No such file or directory
mergify: resuming merge at
mv: rename .git/MERGIFY_CURRENT_HASH to .git/MERGIFY_LAST_MERGED_HASH: No such file or directory
mergify: internal error: reached last hash, but not at ###########
```