getUser on DefaultBuildEngine try calling UserManager.getByFullName if UserManager.get(userName); is null
Adding call to getByFullName would reduce our user mappings. Bitbucket using full name and Quickbuild uses user name.

private User getUser(Repository repository, String committer)
{

User user = userManager.get(userName);
// add these lines
if(user == null)
{
    user = userManager.getByFullName(userName);
}
// done with added lines.

if(user == null)
 {
               user = new User();
               user.setName(userName);
.....

Or adding a global user mapping configuration.