import: allow processing of extra part header during import
As we have a way for extension to add more header, we need a way for them to
actually process them. We add a basic hook points to alter the changeset
(especially extra) before we commit. There would be more to do for a full
featured hooking, but this currently fit my needs.
$ 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
M a
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