Bitbucket Environment Variable RCE

For various versions of Bitbucket, there is an authenticated command injection
vulnerability that can be exploited by injecting environment
variables into a user name. This module achieves remote code execution
as the `atlbitbucket` user by injecting the `GIT_EXTERNAL_DIFF` environment
variable, a null character as a delimiter, and arbitrary code into a user's
user name. The value (payload) of the `GIT_EXTERNAL_DIFF` environment variable
will be run once the Bitbucket application is coerced into generating a diff.

This module requires at least admin credentials, as admins and above
only have the option to change their user name.