Attempt to support versioning/publishing & tagging in separate git branches

[ezze]

Description

This pull request allows passing --since argument to version and changed commands.

Motivation and Context

This is an attempt to support a feature described in #2023. Unfortunatelly, I didn't find another way to do it.

How Has This Been Tested?

Initially I made changes to version and changed commands in Lerna npm packages installed in my project's node_modules directory and tested them by running:

$ lerna changed --since 0.14.1
$ lerna version --since 0.14.1
$ lerna publish --since 0.14.1

Becoming sure they work as I expect, I added these changes to source code and provided a test checking that collectUpdates function is called with passed git commit or tag. Unfortunatelly, this publish command's test is not passed. Being new to Lerna I don't know why passing since is prohibited there. Does any workaround exist?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

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

[evocateur]

The test exists because --since is not allowed for publish or version. This isn't changing, sorry.