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.

diff --cc t/t7501-commit.sh
Simple merge

diff --cc wt-status.c
Simple merge