Option to use tag_filter across all branches

[sabjorn]

added new flag, enable_tag_multibranch, to allow tag_filter option to use all branches of git repository. This is useful for when independent branches are used for release but tags are used for triggering builds.

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[vito]

Hmm, I think the idea makes sense but I'm not sure this will work.

The --single-branch was added as an optimization, but the git log command that we run should still only apply to the configured branch.

Maybe branch should be optional if tag_filter is specified, and if left blank, tags should be queried across all branches, and --single-branch should not be used?

Also, could you add tests?

[sabjorn]

@vito, closing this PR for now because the PR is on a branch which feeds into a CI build for the resource. I'll pop this back once I get these tests written.

[vito]

for anyone following on, this is now tracked in #165