merge: duplicate 'if f in copied' into each branch

merge: branch code into {n1 and n2, n1, n2} top-level cases There are three high-level cases that are of interest in manifestmerge(): 1) The file exists on both sides, 2) The file exists only on the local side, and 3) The file exists only on the remote side. Let's make this clearer in the code. The 'if f in copied' case will be broken up into the two applicable branches in the next patch.

update: remove redundant and misplaced merge tests The "nothing to merge" case is covered by test-merge-default.t. The "uncommitted changes" case is covered by test-merge1.t (and others). The "merge -f" case is covered by test-merge-force.t.