Pytest-benchmark

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* Changed ``--benchmark-warmup`` to take optional value and automatically activate on PyPy (default value is ``auto``).
**MAY BE BACKWARDS INCOMPATIBLE**
* Removed the version check in compare mode (previously there was a warning if current version is lower than what's in
the file).

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* Changed how comparison is displayed in the result table. Now previous runs are shown as normal runs and names get a
special suffix indicating the origin. Eg: "test_foobar (NOW)" or "test_foobar (0123)".
* Fixed sorting in the result table. Now rows are sorted by the sort column, and then by name.
* Show the plugin version in the header section.
* Moved the display of default options in the header section.

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* Add a ``--benchmark-disable`` option. It's automatically activated when xdist is on
* When xdist is on or ``statistics`` can't be imported then ``--benchmark-disable`` is automatically activated (instead
of ``--benchmark-skip``). **BACKWARDS INCOMPATIBLE**
* Replace the deprecated ``__multicall__`` with the new hookwrapper system.
* Improved description for ``--benchmark-max-time``.

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* Tests are sorted alphabetically in the results table.
* Failing to import ``statistics`` doesn't create hard failures anymore. Benchmarks are automatically skipped if import
failure occurs. This would happen on Python 3.2 (or earlier Python 3).

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* Changed how failures to get commit info are handled: now they are soft failures. Previously it made the whole
test suite fail, just because you didn't have ``git/hg`` installed.

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* Added progress indication when computing stats.