copies: compute the exact set of revision to walk

This change make the code clearer by removing the revision queue. It comes

without very noticeable performance impact. However the simpler code will be

easier to update in later changesets.

revision: large amount; added files: large amount; rename small amount; c3b14617fbd7 9ba6ab77fd29

before: ! wall 1.430082 comb 1.430000 user 1.390000 sys 0.040000 (median of 10)

after: ! wall 1.405192 comb 1.410000 user 1.390000 sys 0.020000 (median of 10)

revision: large amount; added files: small amount; rename small amount; c3b14617fbd7 f650a9b140d2

before: ! wall 1.971366 comb 1.970000 user 1.950000 sys 0.020000 (median of 10)

after: ! wall 1.892541 comb 1.890000 user 1.870000 sys 0.020000 (median of 10)

revision: large amount; added files: large amount; rename large amount; 08ea3258278e d9fa043f30c0

before: ! wall 0.252594 comb 0.250000 user 0.240000 sys 0.010000 (median of 38)

after: ! wall 0.240075 comb 0.240000 user 0.240000 sys 0.000000 (median of 40)

revision: small amount; added files: large amount; rename large amount; df6f7a526b60 a83dc6a2d56f

before: ! wall 0.013100 comb 0.010000 user 0.010000 sys 0.000000 (median of 226)

after: ! wall 0.013247 comb 0.010000 user 0.010000 sys 0.000000 (median of 223)

revision: small amount; added files: large amount; rename small amount; 4aa4e1f8e19a 169138063d63

before: ! wall 0.001633 comb 0.000000 user 0.000000 sys 0.000000 (median of 1000)

after: ! wall 0.001670 comb 0.000000 user 0.000000 sys 0.000000 (median of 1000)

revision: small amount; added files: small amount; rename small amount; 4bc173b045a6 964879152e2e

before: ! wall 0.000078 comb 0.000000 user 0.000000 sys 0.000000 (median of 11984)

after: ! wall 0.000119 comb 0.000000 user 0.000000 sys 0.000000 (median of 7982)

revision: medium amount; added files: large amount; rename medium amount; c95f1ced15f2 2c68e87c3efe

before: ! wall 0.207093 comb 0.210000 user 0.210000 sys 0.000000 (median of 47)

after: ! wall 0.201551 comb 0.200000 user 0.200000 sys 0.000000 (median of 48)

revision: medium amount; added files: medium amount; rename small amount; d343da0c55a8 d7746d32bf9d

before: ! wall 0.038462 comb 0.040000 user 0.040000 sys 0.000000 (median of 100)

after: ! wall 0.036578 comb 0.030000 user 0.030000 sys 0.000000 (median of 100)

Differential Revision: https://phab.mercurial-scm.org/D7076