Bug 293932 – export-w3c-test-changes doesn't need to checkout other local branches

• Status: NEW
• Component: Tools / Tests

[Sam Sneddon [:gsnedders]]

```
gsnedders@gsnedders-margot OS3 % ./Tools/Scripts/export-w3c-test-changes -c -g f456176 -b 279200 -d ~/projects/wpt/wpt-export -n gsnedders
Fetching web-platform-tests repository
Cleaning web-platform-tests master branch
Traceback (most recent call last):
  File "/Volumes/gsnedders/projects/Safari/OS3/./Tools/Scripts/export-w3c-test-changes", line 35, in <module>
    sys.exit(test_exporter.main(sys.argv[1:], sys.stdout, sys.stderr))
  File "/Volumes/gsnedders/projects/Safari/OS3/Tools/Scripts/webkitpy/w3c/test_exporter.py", line 479, in main
    test_exporter.do_export()
  File "/Volumes/gsnedders/projects/Safari/OS3/Tools/Scripts/webkitpy/w3c/test_exporter.py", line 378, in do_export
    self.clean()
  File "/Volumes/gsnedders/projects/Safari/OS3/Tools/Scripts/webkitpy/w3c/test_exporter.py", line 289, in clean
    self._git.checkout('master')
  File "/Volumes/gsnedders/projects/Safari/OS3/Tools/Scripts/webkitpy/common/checkout/scm/git.py", line 557, in checkout
    return self._run_git(command)
  File "/Volumes/gsnedders/projects/Safari/OS3/Tools/Scripts/webkitpy/common/checkout/scm/git.py", line 76, in _run_git
    return self.run(full_command_args, **full_kwargs)
  File "/Volumes/gsnedders/projects/Safari/OS3/Tools/Scripts/webkitpy/common/checkout/scm/scm.py", line 73, in run
    return self._executive.run_command(args,
  File "/Volumes/gsnedders/projects/Safari/OS3/Tools/Scripts/webkitpy/common/system/executive.py", line 425, in run_command
    (error_handler or self.default_error_handler)(script_error)
  File "/Volumes/gsnedders/projects/Safari/OS3/Tools/Scripts/webkitpy/common/system/abstractexecutive.py", line 98, in default_error_handler
    raise error
webkitpy.common.system.executive.ScriptError: Failed to run "['git', 'checkout', 'master']" exit_code: 128 cwd: /Volumes/gsnedders/projects/wpt/wpt-export
gsnedders@gsnedders-margot OS3 % git -C /Volumes/gsnedders/projects/wpt/wpt-export checkout master
fatal: 'master' is already checked out at '/Volumes/gsnedders/projects/wpt/web-platform-tests'
```

But we don't actually need to checkout `master` before anything else — we just need to make sure we have the HEAD, index, and working tree in a state that matches the default branch. We could just do something like `git switch -d origin/master` — this also has the advantage of not running `git reset --hard` and destroying any state the user already has in their repo, as it will instead cause an error.

[Radar WebKit Bug Importer]

<rdar://problem/152465036>