In collaborative software development, it is considered to be a best practice to submit code changes as a sequence of cohesive commits, each of which records the work result of a specific development \textit{activity}, such as adding a new feature, bug fixing, and refactoring.
However, rather than following this best practice, developers often submit a set of loosely-related changes serving for different development activities as a \emph{composite commit}, due to the tedious manual work and lack of effective tool support to decompose such a \emph{tangled changeset}.
Composite commits often obfuscate the change history of software artifacts and bring challenges to efficient collaboration among developers.
To encourage \textit{activity-oriented} commits, we propose \textit{SmartCommit}, a graph-partitioning-based interactive approach to tangled changeset decomposition that leverages not only the efficiency of algorithms but also the knowledge of developers.
To evaluate the effectiveness of our approach, we (1) deployed \textit{SmartCommit} in an international IT company, and analyzed usage data collected from a field study with 83 engineers over 9 months; and (2) conducted a controlled experiment on 3,000 synthetic composite commits from 10 diverse open-source projects. Results show that \textit{SmartCommit} achieves a median accuracy between 71–84% when decomposing composite commits without developer involvement, and significantly helps developers follow the best practice of submitting \textit{activity-oriented} commits with acceptable interaction effort and time cost in real collaborative software development.