Add merge-conflict handling to status dashboard

[divmain]

Should mirror rebase dashboard functionality.

A - abort
y - use version from your commit
b - use version from merge source commit

[p3lim]

IMO it should handle more like the inline diff view, with non-single-key shortcuts (so that one can write normally) for choosing ours/theirs. Outlining ours/theirs properly with a theme would be a nice addition, maybe in the gutter.