Before this changes, any branch points results in a copy of the dictionary containing the

copy information. This can be very costly for branchy history with few rename

information. Instead, we take a "copy on write" approach. Copying the input data

only when we are about to update them.

In practice we where already doing the copying in half of these case (because

`_chain` makes a copy), so we don't add a significant cost here even in the

linear case. However the speed up in branchy case is very significant. Here are

some timing on the pypy repository.

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

before: ! wall 1.399863 comb 1.400000 user 1.370000 sys 0.030000 (median of 10)

after: ! wall 0.766453 comb 0.770000 user 0.750000 sys 0.020000 (median of 11)

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

before: ! wall 1.876748 comb 1.890000 user 1.870000 sys 0.020000 (median of 10)

after: ! wall 1.167223 comb 1.170000 user 1.150000 sys 0.020000 (median of 10)

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

before: ! wall 0.242457 comb 0.240000 user 0.240000 sys 0.000000 (median of 39)

after: ! wall 0.211476 comb 0.210000 user 0.210000 sys 0.000000 (median of 45)

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

before: ! wall 0.013193 comb 0.020000 user 0.020000 sys 0.000000 (median of 224)

after: ! wall 0.013290 comb 0.010000 user 0.010000 sys 0.000000 (median of 222)

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

before: ! wall 0.001673 comb 0.000000 user 0.000000 sys 0.000000 (median of 1000)

after: ! wall 0.001677 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 0.000119 comb 0.000000 user 0.000000 sys 0.000000 (median of 8023)

after: ! wall 0.000119 comb 0.000000 user 0.000000 sys 0.000000 (median of 7997)

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

before: ! wall 0.201898 comb 0.210000 user 0.200000 sys 0.010000 (median of 48)

after: ! wall 0.167415 comb 0.170000 user 0.160000 sys 0.010000 (median of 58)

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

before: ! wall 0.036820 comb 0.040000 user 0.040000 sys 0.000000 (median of 100)

after: ! wall 0.035797 comb 0.040000 user 0.040000 sys 0.000000 (median of 100)

The extra cost in the linear case can be reclaimed later with some extra logic.