mergestate: extract a base class to be shared by future memmergestate
This extracts a new base class from `mergestate` and leaves all the
vfs-touching code in `mergestate`.
Differential Revision: https://phab.mercurial-scm.org/D9039
{
"version": 1,
"project": "mercurial",
"project_url": "https://mercurial-scm.org/",
"repo": "..",
"branches": ["default", "stable"],
"environment_type": "virtualenv",
"show_commit_url": "https://www.mercurial-scm.org/repo/hg/rev/",
"benchmark_dir": "benchmarks",
"env_dir": "../.asv/env",
"results_dir": "../.asv/results",
"html_dir": "../.asv/html"
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