Ignore std error on version discovery

git may print warnings out that can confuse the parsing, and std error
could never provide a trustworthy errorn number anyway. (In my case, it
was providing a tag name which provided a valid but undesirable match,
so I was ending up with the wrong version number)

Change-Id: I61bccacfff20fd4afd8ac8d73b69e2deab50d1c7
Merge branch 'et_setup_version' of git://github.com/jgonzalezmendez/buildbot