### Purpose
This PR changes behaviour of syncthing related to `receive-only`
folders, which I believe to be a bug since I wouldn't expect the current
behaviour. With the current syncthing codebase, a file of a
`receive-only` folder that is only modified locally can cause the
creation of a `.sync-conflict` file.
### Testing
Consider this szenario: Setup two paired clients that sync a folder with
a given file (e.g. `Test.txt`). One of the clients configures the folder
to be `receive-only`. Now, change the contents of the file for the
receive-only client **_twice_**.
With the current syncthing codebase, this leads to the creation of a
`.sync-conflict` file that contains the modified contents, while the
regular `Test.txt` file is reset to the cluster's provided contents.
This is due to a `protocol.FileInfo#ShouldConflict` check, that is
succeeding on the locally modified file.
This PR changes this behaviour to not reset the file and not cause the
creation of a `.sync-conflict`. Instead, the second content update is
treated the same as the first content update.
This PR also contains a test that fails on the current codebase and
succeeds with the changes introduced in this PR.
### Screenshots
This is not a GUI change
### Documentation
This is not a user visible change.
## Authorship
Your name and email will be added automatically to the AUTHORS file
based on the commit metadata.
#### Thanks to all the syncthing folks for this awesome piece of
software!
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