it rebuilds "stage0 tool compiletest" even if you only change the compiler

[est31]

Do the following: ./x.py test --stage 1 src/test/run-pass --test-gs i128

Then change something in the compiler, e.g. a comment in librustc.

Then invoke the command again. You'll see that not just the compiler but also compiletest is being recompiled even though it doesn't have to.

cc @Mark-Simulacrum