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dirstate-v2: Rename Header to Root, move it to the end of the data file

Authored by SimonSapin.

Description

dirstate-v2: Rename Header to Root, move it to the end of the data file

Now that they don’t have to be at the start, a given data file may contain
multiple "roots". A docket only points to one of them, and previous ones
are left unused to allow allow append-only in-place writing to an existing
data file.

Differential Revision: https://phab.mercurial-scm.org/D11090