Support skipping merge commits

[korthout]

Is your feature request related to a problem? Please describe.
Repositories commonly update pull requests with changes from the base using a merge commit. But, the action does not support the cherry-picking of merge commits. So, it fails to backport.

Describe the solution you'd like
We can allow users to decide how to deal with merge commits using a new input. This input defaults to the current behavior and allows users to skip merge commits instead.

Describe alternatives you've considered
Alternatively, the mainline flag can be used to decide how to cherry-pick the merge commit. We consider this a separate feature request which is covered by #341. If you have a need for that, please let us know there.

[korthout]

Released in v1.4.0, v1.4, and v1, which now support the new merge_commits input.

FYI @tdonohue @houserx-jmcc

[korthout]

@dlavrenuek I think you may also be interested in this new input.

[tdonohue]

Thank you so much @korthout ! I just got around to testing this new merge_commits: 'skip' option and it works perfectly for our needs. Merge commits are skipped automatically and not included in the port PR.