[MERGE][bug #300289] Avoid unnecessary work

[John Arbash Meinel]

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I haven't fully understood why, but when branching from a stacked branch
into a standalone branch, it turns out that this "set.difference_update"
was a significant portion of the overall time.

Adding this shortcut drops the time for "bzr branch stacked standalone"
from 3m9s down to 1m45s (approx 40% of the total time.)

My guess is that we are doing something like has_key() on the target
index (which is being built). (I see 203k calls to
BTreeBuilder.iter_entries().) Also, in a bit of testing that I did, it
seems that set.difference_update() *always* iterates the full target,
even when source is small.

For example "small_set.difference_update(large_set)" is relatively slow.
(Of course, calling it 172k times doesn't help.)

I'm still investigating more fixes for stacked branches, but this was a
small patch that made a big difference.

John
=:->
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