Add pre-conflict merge screen

[skjnldsv]

cc @szaimen @nimishavijay

[szaimen]

Looks like a good proposal to me in general 👍

However I would probably name the first one Keep all files instead of Keep both files

And we should probably add icons to the buttons?

Also cc @jancborchardt @marcoambrosini

[marcoambrosini]

I think that the let me choose file by file should probably be the default option.