Version 1.2.0

This version adds some new options:

• --show-commit: Display only the SHA-1 of the merge commit. Exit with a nonzero exit code if the commit was not merged via a merge commit. This option is meant to make it convenient to call git when-merged from scripts.
• --show-branch: Display all of the commits that were merged at the same time as the specified commit (e.g., the whole pull request branch). This option also interacts nicely with --log and --visualize.
• --describe and --describe-contains: Describe the merge commit in terms of nearby tags (i.e., pass it through git describe or git describe --contains).