Keep the Git helper next to build_version.rs as build_git.rs instead of a
path-mapped build_support module. Watch refs/heads and refs/tags rather
than all of refs/, so fetch, push and stash no longer rerun the core build
script, and cover that in the invalidation test. Shorten the contributor
note to the behavior a contributor sees.
Signed-off-by: Shiju <shiju@nvidia.com>
