Merge branch 'ss/commit-dry-run-resolve-merge-to-no-op'

"git commit --dry-run" reported "No, no, you cannot commit." in one case where "git commit" would have allowed you to commit, and this improves it a little bit ("git commit --dry-run --short" still does not give you the correct answer, for example). This is a stop-gap measure in that "commit --short --dry-run" still gives an incorrect result. * ss/commit-dry-run-resolve-merge-to-no-op: wt-status.c: set commitable bit if there is a meaningful merge.