Do not reject detached HEAD when publishing a canary release

[evocateur]

Motivation and Context

To facilitate --canary publishes in CI, we shouldn't break lerna publish when it detects a detached HEAD when canary is enabled. We also need to avoid calling git symbolic-ref because it breaks in detached HEAD situations (which is going to be the case for a vast majority of CI environments). I renamed the GitUtilities method isDetachedHead() to better express its intent.

I've also clarified some of the internal flag parsing, creating a new gitEnabled instance property that better describes the intent behind !(this.flags.canary || this.flags.skipGit). I was able to simplify some of the conditionals as a result.

This fixes #699

How Has This Been Tested?

I reordered the appropriate ChildProcessUtilities.execSync stubs, and added a new test for a complicated set of configuration to improve coverage.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[lock]

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