Allow update from contribution target unless you're on it #15309
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Closes #15299
Description
This is a follow-up to #14199 which enabled updating from the contribution target's default branch instead of the default branch of the origin remote. We seem to have forgotten to update the logic for determining when such a merge is possible. Previously we deemed it possible as long as we weren't on the default branch (since the default branch was always the source of the merge) whereas now we need to look at whether we're currently on the default branch of the contribution target or not (which may be the default branch of the origin remote).
Release notes
Notes: [Fixed] Enable update from upstream default branch when contributing to a fork