mergeview: add commands to apply the merge.

mergeview: add commands to apply the merge.

If :A-m has a numeric arg, then it now behaves differently and often
edits the merge rather than finding another:

-ve: discard the patch and just keep the original.
0 : if no conflicts remain, apply the patch
1 : discard the original and just keep the "after" part.

9 : if there is a selected merge, cut the path and leave the original
    if there is no selected merge, paste the recently cut diff,
     using the selection as the original.

Signed-off-by: NeilBrown <neil@brown.name>