largefiles: update should only create a .orig backup of a largefile once
A .orig of a standin after the update do that a .orig of the actual largefile
is created. The .orig standin was however never removed again and the largefile
.orig was thus overwritten again and again.
The fix: remove the standin .orig when it is used.
merge: let manifestmerge emit 'keep' actions when keeping wd version
Such a 'keep' action will later be the preferred (non)action when there
is multiple ancestors. It is thus very convenient to have it explicitly.
The extra actions will only be emitted in the case where the local file has
changed since the ancestor but the other hasn't. That is the symmetrical
operation to a 'get' action.
This will create more action tuples that not really serve a purpose. The number
of actions will however have the number of changed files as upper bound and it
should thus not increase the memory/cpu use significantly.