Stage files as you complete them.

When the last conflict in a file is resolved, or when the whole file is resolved at once, display an overlay that offers to save and stage the file in its resolved state.

When the last resolved file is staged, show a completion message and remove all merge conflict panes.

Add a "quit" option to the `MergeConflictsView` to trigger an exit to conflict-resolution mode even if there are still conflicts to resolve. Show a message and remove all merge conflict panes. (#3)