Well, it is kind-of possible for Ruby/Rails. I half-built such a system once. Here's what you do:
* Gather coverage information when running tests or compiling assets.
* Use that to generate Rakefiles for running tests / compiling assets that have dependencies on the source files they would touch.
* Run the tests once, recording information about the git status of each dependency.
* When the git status of a file changes (i.e. you've just fetched some changes), mark the dependency as dirty
Even better, it you're gathering coverage data and know the test status for each commit, then not only can you show the commit that broke the build, you can show the particular part of the diff that broke the build.
I think you mean "magic".
That is, this is not possible :) Not that I wouldn't like it to be.