[MERGE] Make merge more than 2x faster

[Aaron Bentley]

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Hi all,

This patch improves the speed of merge by
- - avoiding a pointless round of conflict detection
- - avoiding duplicate_entry detection when no files have been added
- - avoiding checking whether the base revision is an ancestor if it was
  selected via least-common-ancestor
- - using set_parent_trees instead of add_parent
- - using dirstate revision trees where possible
- - caching revision trees for later use

Combined, for my example, they reduce total execution time from ~3.2
seconds to ~1.5 seconds.

Aaron
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