reduce log spew during build
reduce log spew during build

Summary:
to compute the build id, we shell out to `git` and then `hg`. in an hg repo, the git one is expected to fail, so the output is spew.

instead, we now only output the failures if they both fail.

Reviewed By: panagosg7

Differential Revision: D24478519

fbshipit-source-id: 0e6b9049997844f761c677b5ad908bb5661f29fb

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow