contrib/asv.conf.json

context: move _manifest from committablectx to workingctx

committablectx had a _manifest implementation that was only used by the derived
workingctx class. The other derived versions, like memctx and metadataonlyctx,
define their own _manifest functions.

Let's move the function down to workingctx, and let's break it into two parts,
the _manifest part that reads from self._status, and the part that actually
builds the new manifest. This separation will let us reuse the builder code in a
future patch to answer _buildstatus with varying status inputs, since workingctx
has special behavior for _buildstatus that the other ctx's don't have.

{
    "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"
}