"Squash commits" option and "Squash commit message" aren't stored for merge requests
### Summary
I just realized that the state of the "Squash commits" checkbox and the text entered into the resulting "Squash commit message" input are not saved for merge requests.
I think this worked before, so this might be a regression. Potentially it also depends on state (maybe it only works when "merge when pipeline uscceds" is activated?).
### Steps to reproduce
* Visit a merge request
* enable "Squash commits" checkbox and add some text in the "Squash commit message" input
* Result:

* Now reload the page
* Result 1 - checkbox unchecked:

* Result 2 - commit message deleted and replaced with the default:

### Example Project
Observed at https://gitlab.com/inkscape/inkscape/merge_requests/811
### What is the current *bug* behavior?
State of checkbox and content of text input not saved
### What is the expected *correct* behavior?
The state of the checkbox and the content of the text input should be saved.
This would allow a workflow, where a project maintainer sets the flags as desired and modifies the commit message as needed as a step of the review process.
Current behavior forces maintainers to do all that when merging the MR. Also it requires a little ridiculous review comments like "looks good if minor issue X is fixed; should only be merged squashed and after changing commit message to ..."
issue