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revset: optimize baseset.__sub__ (issue4313)

dd716807fd23 regressed performance of baseset.__sub__ by introducing
a lazyset. This patch restores that lost performance by eagerly
evaluating baseset.__sub__ if the other set is a baseset.

revsetbenchmark.py results impacted by this change:

revset #6: roots(0::tip)
0) wall 2.923473 comb 2.920000 user 2.920000 sys 0.000000 (best of 4)
1) wall 0.077614 comb 0.080000 user 0.080000 sys 0.000000 (best of 100)

revset #23: roots((0:tip)::)
0) wall 2.875178 comb 2.880000 user 2.880000 sys 0.000000 (best of 4)
1) wall 0.154519 comb 0.150000 user 0.150000 sys 0.000000 (best of 61)

On the author's machine, this slowdown manifested during evaluation of
'roots(%ln::)' in phases.retractboundary after unbundling the Firefox
repository. Using `time hg unbundle firefox.hg` as a benchmark:

Before: 8:00
After:  4:28
Delta: -3:32

For reference, the subset and cs baseset instances impacted by this
change were of lengths 193634 and 193627, respectively.

Explicit test coverage of roots(%ln::), while similar to the existing
roots(0::tip) benchmark, has been added.

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