Allow arc to parse `svnlook` diffs from moves/copies

Summary: Ref T4697. This is similar to the existing stuff, but `svnlook diff ...` can also produce a "Copied" header. Currently, we choke on it in Herald when running pre-commit rules.

Test Plan: Added and ran tests. In the next diff, used this in a real system.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4697

Differential Revision: https://secure.phabricator.com/D8653
Strip comments during `arc diff --verbatim` workflow before submitting to Phabricator

Summary: Fixes T4649. The issue in that task is caused because we're submitting a block of text including comments. We've probably been doing this for a long time, but maybe were more liberal in parsing before. Instead, strip them.

Test Plan: Ran `arc diff --verbatim` and got "Dxxx" prefilled correctly.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4649

Differential Revision: https://secure.phabricator.com/D8658
Fix escaping of bookmarknames properly for `arc patch`

Summary:
Fixes T4596. I misunderstood this issue and D8512 was not correct. Specifically:

  - The `hg log` needs to be escaped, since otherwise "arcpatch-x" is interpreted as a revset.
  - The `hg update` does not need to be escaped, since updating to a revset doesn't make sense and the command never treats its argument as a revset.
  - The `hg bookmark` does not need to be escaped, for similar reasons.

Test Plan:
  - Ran these commands in isolation and got sensible, consistent results.
  - Ran `arc patch` several times in a row and got proper bookmark names.

Reviewers: btrahan, durham, rvanvelzen

Reviewed By: rvanvelzen

Subscribers: epriestley

Maniphest Tasks: T4596

Differential Revision: https://secure.phabricator.com/D8661
Add coverage reports to py.test runner

Summary: Of note this now forces the pytest-cov plugin to be installed unless they turn it off.

Test Plan:
```
../arcanist/bin/arc unit

COVERAGE REPORT
     97%     changes/models/test.py
    100%     tests/changes/api/serializer/models/test_testcase.py
    100%     changes/api/serializer/models/testcase.py
 UNIT OKAY  No unit test failures.
Updated an existing Differential revision:
        Revision URI: https://tails.corp.dropbox.com/D43387

Included changes:
  M       changes/api/serializer/models/testcase.py
  M       changes/models/test.py
  M       tests/changes/api/serializer/models/test_testcase.py
```

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8667
Merge branch 'master' of http://github.com/facebook/arcanist