lerna publish --canary=${my_prerelease_tag} doesn't work

[achingbrain]

When I use the publish command like so:

lerna publish --canary=staging

I would expect changed modules to be published with versions similar to:

Changes:
  - my-module: 1.0.0 => 1.0.0-staging.dd3c916f

Instead it tries to use the value passed to the --canary option to work out a range of git commits:

$ lerna publish --yes --canary=staging
lerna info version 2.1.2
lerna info versioning independent
lerna info canary enabled
lerna info Checking for updated packages...
lerna info Comparing with staging^..staging.
lerna ERR! initialize Error: fatal: bad revision 'staging^..staging'

Expected Behavior

The value passed to the canary flag should appear in the published version string.

Current Behavior

The value passed to the canary flag is used as a git commit id which explodes.

Steps to Reproduce (for bugs)

1. Pass a value to the canary flag

Your Environment

Executable	Version
lerna --version	2.1.2
npm --version	5.4.0
yarn --version	n/a
node --version	6.9.1

OS	Version
NAME	VERSION
macOS Sierra	10.12.5

[lock]

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