dim: git_fetch_helper

Old versions of git fetch exited with 128 when there was nothing to
fetch. But ignoring all return values is bad as Chris recently found
out, since then rebuilding drm-tip might go back to older branch
versions if the fetch fails for some reason.

I also noticed that current git (I have 2.11 here) don't exhibit this
anymore.

v2: s/exit/return/ to give callers a chance to do something with this.
We use set -e, so same result (Jani).

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>