gh pr create: Support Git's @{push} revision syntax for determining head ref#10513

BagToad merged 40 commits intotrunkcli/cli:trunkfrom

kw/575-detect-push-target-for-local-branches-without-upstream-configurationcli/cli:kw/575-detect-push-target-for-local-branches-without-upstream-configurationCopy head branch name to clipboard