Add since option to add command

[juanrgm]

This option is very useful when you only want one changeset by publish, ex:

git add packages/one
git commit -m "add package one"
git add packages/two
git commit -m "add package two"
changeset add --since origin/main

[changeset-bot]

🦋 Changeset detected

Latest commit: 0773fe0

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@changesets/cli	Minor

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