bundle: introduce a listkey handler
This makes it possible to handle listkeys-related data stored in a bundle2.
There are no producers yet.
$ hg init
$ echo a > a
$ hg add a
$ hg commit -m test
Do we ever miss a sub-second change?:
$ for i in 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20; do
> hg co -qC 0
> echo b > a
> hg st
> done
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M a
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M a
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M a
M a
M a
M a
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M a
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M a
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