Feedback for squash merges

This is a placeholder ticket to gather feedback for the squash merge labs feature. The reason for a separate feedback ticket is to not spam people in the original ticket who just want the feature but don't want to be involved in further discussion.

What currently works

• Pull request merge dialog contains a dropdown that can be used to choose between having a merge commit and just squashing all your commits on top of the destination.
• The default commit message changes based on how one wants to do the merge. In the case of squash merges, we use a combination of the commit messages of all the commits being squashed. Obviously, the user should probably edit this commit message, but the default is a good start.

Known missing functionality

• The merge dialog in the branch details page, branch list page and compare page does not yet support squashing. These will be rolled out in the coming weeks. (done)
• Mercurial support for squash merges is planned, but will be implemented after all the merge dialogs are fixed. (we're planning on supporting obsolescence markers for this, so it'll be split into a separate beta feature later).
• API support is missing.
• Public documentation has not been updated yet (obviously this should be the last task).