`x.py build --stage 1 src/libstd --target $T` rebuilding rustc crates when not necessary

[japaric]

Logs of two consecutive x.py build calls that rebuild the rustc crates but not the crates "below" std. (Grep for $ to see the commands)

cc @alexcrichton

[alexcrichton]

Suspicious:

stale: /shared/rust/checkouts/musl/src/liballoc_jemalloc/../jemalloc/.git/ORIG_HEAD -- 1480531948.379850662s vs 1480530784.555870554s

I think that'd trigger a rebuild of everything? Maybe this is just alloc_jemalloc's build script being too eager about cargo:rerun-if-changed. Could you try tweaking that build script and see if it fixes the problems here?