target_commitish in GitHub Releases has two different meanings:
* Target commit that a new associated tag will be created on
* Target branch to compute how many "commits to since this release"
target_commitish is usually the default branch (aka master), because the
distance between the release and the latest branch is the primary concern.
Before this change, target_commitish was left to be a hash and the
feature of GitHub Releases that shows how much time passed since the
release is ruined, because no tracking branch is given.
By this change, target_commitish is changed to be the default branch
given in the configuration (options.branch) via edit a release API after
the release and tag are created via crate a release API.
f148a61339