Test coverage tool that shows call graph of untested new code from a merge

I'm looking for a test coverage tool that can show the call path/call graph of new untested code that came from a merge.

Example: I have a git repo (on github or bitbucket) and a test suite that covers X% on master. Other developer wants to contribute and do a fork and a pull request as usual, but he/she didn't included the tests (for many reasons). Then the tool show the untested call graphs that only came after the integration of this new code.

No specific stack involved besides the git, just wanna know if there is a tool available.