test: add test for a former race resulting in bad dirstate
In 6.0 this used to result in the size being stored in the dirstate is wrong.
This was fixed by other change to the mtime gathering logic.
Differential Revision: https://phab.mercurial-scm.org/D11749
rhg: centralize index header parsing
Centralize index header parsing, parse the generaldelta flag,
and leave breadcrumbs to relate the code to python.
Differential Revision: https://phab.mercurial-scm.org/D11881