Fix tag ref for Bitbucket Cloud

[riddler7]

Changes

The Bitbucket Cloud webhook payload does not include refs/tags in the branch name. This means the using something like pipelinesascode.tekton.dev/on-target-branch: "[refs/tags/*]" does not work. This PR sets the branch name to refs/tags/tag when the change type is tag.

There is a slight concern around backwards compatibility if someone was using something like pipelinesascode.tekton.dev/on-target-branch: "[1.*]", as the source_branch variable will no longer be the tag, but refs/tags/tag. When testing this, while the tag was matched for the pipeline, the source_branch was empty.

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