merge: remove unnecessary matcher checks As part of changing manifest.diff to accept a matcher, a previous patch added matcher calls to each location in merge.manifestmerge that tested if 'x in mf' to maintain the same behavior as before. After analyzing it further, this matcher call isn't needed, and in fact hurts future patches ability to use the matcher here. Basically, all these 'if x in mf' checks were checking if a matched file's copy source was in the matcher as well. This meant if you passed a matcher for just file foo, it would not return file bar even if foo was a copy of bar. Since manifestmerge cares about copy information, let's allow all lookups of copy sources. We also update one spot with a 'is not None' check, since it wasn't obvious that the value could sometimes be None before, which broke when we called matcher(None). A future patch adds matcher optimizations to manifestmerge which causes this code path to get covered by existing tests.

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